Femtosecond Laser-Induced Underwater Superoleophobic Surfaces

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Femtosecond Laser-Induced Underwater Superoleophobic Surfaces with Reversible pH-Responsive Wettability Jingzhou Zhang, Jiale Yong, Qing Yang, Feng Chen, and Xun Hou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04069 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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Femtosecond Laser-Induced Underwater Superoleophobic Surfaces with Reversible pH-Responsive Wettability

Jingzhou Zhang1,3, Jiale Yong1,3, Qing Yang2,3, Feng Chen1,3,* and Xun Hou1 1State

Key Laboratory for Manufacturing System Engineering and Shaanxi Key Laboratory of Photonics

Technology for Information, School of Electronics & Information Engineering, Xi’an Jiaotong University, Xi’an, 710049, PR China 2School 3The

of Mechanical Engineering, Xi’an Jiaotong University, Xi’an, 710049, PR China

International Joint Research Laboratory for Micro/Nano Manufacturing and Measurement Technologies,

Xi’an Jiaotong University, Xi’an, 710049, PR China

*Corresponding author: [email protected]

Abstract Wettability-switchable surfaces have become a research hotspot because they can exhibit different superwetting states. In this paper, the copper surfaces with pH-responsive underwater oil wettability were prepared by femtosecond laser treatment and subsequent chemical modification. The resultant surfaces showed underwater superoleophobicity in alkaline solutions but quasi-superoleophilicity in acidic solutions. The contact angles of an underwater oil droplet on the resultant surfaces could be reversibly tuned between 157° and 12° by changing the pH of aqueous solutions. Such switchable wettability is ascribed to the modification of the alkyl and carboxylic acids groups on the laser-structured surfaces. The as-prepared surfaces have both oil-resistance and oil-collection abilities by selectively showing underwater superoleophobicity and superoleophilicity. The smart surfaces with pH-responsive oil wettability will have important applications in controlling the oil behavior in water. Keywords: underwater superoleophobicity, underwater superoleophilicity, femtosecond laser, pH-responsive wettability, switchable wettability

Introduction Underwater superoleophobic surfaces with oil contact angle (OCA) greater than 150° in water have attracted considerable attention owing to their broad applications in anti-oil surfaces, lab-on-chip devices, tissue engineering, microreactors, and oil/water separation devices.1-6 After analyzing the anti-oil ability of the fish scales and the clam shells, Jiang et al. revealed that such underwater anti-oil property is ascribed to

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the combination of rough surface microstructures and hydrophilic chemical composition.7,8 Different from the superoleophobic surfaces, the underwater superoleophilic surfaces show OCA smaller than 10° and are the combination of rough microtopography and hydrophobic chemical composition.9 The underwater superoleophilic surfaces also have important applications in bioadhesion, oil collection, and the absorption of waste oil.10-14 With the development of stimuli-responsive materials, intense attention has been focused on the surfaces with switchable wettability.15-23 Until now, a large number of wettability-switchable surfaces have been prepared. For instance, Sun et al. showed a thermal-responsive wettability on N-isopropylacrylamide surface, whose chemical composition can be changed in a narrow temperature.24 After building rough surface microstructures, the as-prepared surface showed superhydrophilicity at 25℃ and superhydrophobicity at 40℃. Liu et al. reported an electrical-responsive wettability on the polypyrrole nanotube arrays, resulted from the cooperation of the electrical double layer and the electrochemically tunable doping.25 Tian et al. coated a stainless steel mesh with ZnO nanorods array.26 Due to the photo-responsive wettability of ZnO, underwater superoleophobicity and superoleophilicity could be switched through alternate UV irradiation and dark storage. Cheng et al. obtained a smart superhydrophobic surface on shape-memory polymer (SMP) arrays.27 Since the surface microstructures could be reversibly changed from rice-leaf-like structures to lotus-leaf-like structures, the wettability of the resultant surface were reversibly switched between superhydrophobic anisotropic and isotropic states. As an emerging technology, femtosecond laser microfabrication is successfully applied in preparing superwetting surfaces because of many unique advantages, including precise ablation threshold, excellent controllability, and negligible heat-affected zone.28-30 Various superwetting surfaces have been fabricated by femtosecond laser processing.31-38 For example, Stratakis et al. built hierarchical superhydrophobic micro/nanostructures on Si by femtosecond laser.39 Wu et al. prepared micropore arrays on an aluminum foil by femtosecond laser perforating and endowed the foil with underwater superoleophobicity and oil/water separation function.40 Yong et al. used femtosecond laser ablation to fabricate slippery surfaces with porous network microstructures on various polymer surfaces.41 The above-mentioned surfaces mainly present a single and fixed superwetting state once the surface micro/nanostructures are formed. To the best of our knowledge, the femtosecond laser-structured surfaces with stimuli-responsive superwettability are still rarely reported. In this paper, we prepared an underwater superoleophobic surface with pH-responsive wettability by femtosecond laser ablation and chemical modification. A copper plate was ablated by laser to generate regular hierarchical micro/nanostructures and then was assembled with responsive chemical groups. The resultant surface showed superoleophobicity in alkaline solutions (pH > 11) and quasi-superoleophilicity in acidic solutions (pH < 3). These two superwetting states could be reversibly switched for many cycles by changing the water pH. The switchable wettability resulted from the combination of the laser-induced hierarchical


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microstructures and the reversible chemical transformation on the sample surface.

Experimental section Figure 1 depicts the preparation process of the wettability-switchable surfaces. Copper plate was chosen as the substrate due to its good anti-corrosion property. Firstly, the polished copper plates (99.9%) with the thickness of 0.3 mm were cleaned by acetone, alcohol, and deionized water in an ultrasonic bath for 5 min each time (Figure 1a). Secondly, the samples were mounted on a three-dimensional translation stage and ablated by the focused laser beam (center wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 1 kHz) to form hierarchical rough surface micro/nanostructures (Figure 1b). The laser comes from a regenerative amplified Ti:sapphire laser system (Coherent, Libra-usp-he). The laser beam with the power of 15 mW was focused onto the sample surface by an objective lens (20×, NA = 0.40). Different microstructures (Figure S1, Supporting Information) and underwater oil wettabilities (Figure S2, Supporting Information) could be obtained on the copper surfaces by using different ablating distance during the line-by-line scanning process.42 To achieve great structure-enhanced wettability by building highly uniform hierarchical structures, the main laser scanning speed and the interval of scanning lines were 2 mm/s and 2 µm, respectively, so the average distance of the femtosecond laser-ablated points was 2 μm.36 Thirdly, a layer of gold (thickness of ~30 nm) was sputtered onto each sample surface by an ion sputter (Hitachi, E-1045) to further improve the corrosion resistance (Figure 1c).16,43 Then, the samples were soaked in the mixed solution (1M HS(CH2)10COOH and 1M HS(CH2)9CH3) at room temperature for 12 hours. Finally, the modified samples were taken out of the solution and then dried in vacuum oven at 100 ℃ for 2 hours (Figure 1d).

Figure 1. Schematic illustration of the fabrication process of the wettability-switchable surfaces. (a) A polished flat copper plat. (b) “Line-by-line” laser scanning process to generate surface microstructures on the copper surface. (c) Coating a gold layer on the laser-irradiated surface. (d) HS(CH2)10COOH and HS(CH2)9CH3 modification to endow the surface with switchable chemical groups. Golden color denotes the layer of gold.

The surface morphology of samples were measured by a Quanta 250 scanning electron microscope (SEM, FEI, America) and a Flex SEM 1000 scanning electron


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microscope (HITACHI, Japan). The three-dimensional and the cross-sectional profiles of the sample surfaces were observed by a LEXT-OLS4000 laser confocal microscope (Olympus, Japan). The crystal structures of the laser-ablated copper plates were carried out by a powder X-ray diffractometer (XRD, Rigaku, Smartlab, Japan), using Cu Ka radiation (40 kV and 30 mA). The chemical compositions of the original laser-ablated surface and the modified surface were characterized by an X-ray photoelectron spectroscopy (XPS, Escalab Xi+, Thermo Fisher, America). A JC2000D contact-angle system (Powereach, China) was used to measure the contact angles (CAs) and the sliding angles (SAs) of a water droplet (6μL) and an underwater oil droplet (8 μL) on the sample surfaces. 1,2-Dichloroethane was used as the main test oil. Average values of CA and SA were obtained by measuring three different positions on a same surface. The aqueous solutions with different pH were prepared by diluting hydrochloric acid and potassium hydroxide solutions, respectively. The pH value was monitored by a CT-6020A pH meter (Kedida, China).

Results and Discussion Figure 2a-d shows the SEM images of the copper surface before and after femtosecond laser treatment. The surface of the original polished copper is very smooth (Figure 2a), but microstructures are created on the copper surface by femtosecond laser ablation (Figure 2b). Periodic micro-craters and micro-spikes uniformly distribute on the laser-ablated Cu surface, with the period of 10 µm (Figure 2b). The depth of the micro-craters and the height of the micro-spikes are about 2 µm and 0.8 µm, respectively (Figure 2c and Figure S3 in the Supporting Information). The surfaces of both the micro-craters and the micro-spikes are assembled with a large number of nanoparticles (Figure 2d). The diameter of the nanoparticles ranges from tens to hundreds nanometers. The XRD pattern of the laser-treated surface is shown in Figure S4 (Supporting Information). The peaks at 43.4°, 50.4°, and 74.1° correspond to the Cu (111), Cu (200), and Cu (220), respectively. It demonstrates that the crystal structure of the copper plate does not change after femtosecond laser treatment. The bare flat copper surface shows ordinary oleophobicity in water with an OCA of 117 ± 3° (Figure 2e). After femtosecond laser ablation, the OCA of an underwater oil droplet on the sample surface reaches up to 156 ± 1° in water, showing underwater superoleophobicity of the laser-structured surface (Figure 2f).


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Figure 2. Microstructure and underwater oil wettability of the copper surfaces. (a) SEM image of the polished flat copper surface. (b-d) SEM images of the laser-irradiated copper surface. (e,f) Images of an oil droplet on (e) the polished flat copper surface and (f) the laser-induced rough copper surface in water.

The laser-structured samples were modified with -COOH and -CH3 groups by immersing in the mixed solution. The chemical modification treatment did not change the surface morphology of the laser-induced microstructures (Figure S5, Supporting Information). Chemical change is investigated by an XPS. Figure 3a shows a wide spectrum of the copper surfaces before and after chemical modification. High resolution spectra of Cu 2p, Au 4f, S 2p, and O 1s are detailedly depicted in Figure S6 (Supporting Information). The new Au and S signals are detected, indicating that the gold layer and the –HS groups are successfully coated onto the laser-ablated surface. The C 1s spectra of the original laser-ablated surface can be curve-fitted into peaks of C-C/C-H moieties (284.6 eV) (Figure 3b). After chemical modification, the peaks of C-O (285.4 eV) and C-OO- (288.9 eV) can be curve-fitted from the spectra (Figure 3c), so the functional -COOH groups are successfully modified onto the laser-treated surface.


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Figure 3. (a) XPS survey spectrum of the original laser-ablated surface and the modified rough surface. C 1s XPS spectra with fitting curves of (b) the original laser-ablated surface, and (c) the modified rough surface.

Figure 4a shows an acidic aqueous droplet (pH = 2) on the resultant sample. The measured CA is 144°, revealing quasi-superhydrophobicity of the modified rough surface. By constrast, an alkaline aqueous droplet (pH = 12) on such surface shows superhydrophilicity with a water CA of 10° (Figure 4b). That is, the wettability of the sample surface can be tuned between quasi-superhydrophobicity and superhydrophilicity in air. Regarding the underwater oil wettability, when an oil droplet is put on the surface in acidic solution (pH = 2), the OCA is only 12° (Figure 4c), so the surface exhibits underwater quasi-superoleophobilicity. In the alkaline solution with pH = 12, the sample changes to underwater superoleophobicity with OCA of 157° (Figure 4d). Interestingly, after further cleaned in deionized water and dried, the resultant surface can regain its quasi-superoleophilicity in acid solution (pH = 2). As shown in Figure 4e, such a reversible switching between underwater quasi-superoleophilicity and superoleophobicity can be repeated many times without the decline of the pH responsivity.


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Figure 4. (a) An acidic aqueous droplet (pH = 2) on the as-prepared surface in air. (b) An alkaline aqueous droplet (pH = 12) on the resultant surface in air. (c,d) An oil droplet on the as-prepared surface (c) in the acidic solution with pH = 2 and (d) in the alkaline solution with pH = 12, respectively. (e) Reversible switching between the underwater superoleophobicity and quasi-superoleophilicity by tuning the solution pH.

To further study the relationship between underwater oil wettability and the pH of aqueous solutions, the oil wettability of the as-prepared surface in the aqueous solutions with different pH is investigated. It can be seen in Figure 5 that the OCA increases as the solution pH increases, which is different from the flat and the orginal laser-ablated copper surface without modification (Figure S7, Supporting Information). The obtained OCA values are 12° (pH = 2, Figure 5a), 42° (pH = 4, Figure 5b), 54° (pH = 6, Figure 5c), 47° (pH = 7, Figure 5d), 69° (pH = 8, Figure 5e), 145° (pH = 10, Figure 5f), 152° (pH = 11, Figure 5g), and 157° (pH = 12, Figure 5h), respectively, revealing the transformation from underwater quasi-superoleophilicity to superoleophobicity with increasing pH. The surface exhibits high oil-adhesion with the oil sliding angle (OSA) of 90° in acid or even weak alkalinity aqueous solutions (pH ˂ 11) (Figure 5i). As the pH increases from 10 to 11, the OSA sharply decreases from 90° to 6°. When the pH is larger than 12, the OSA is less than 1°, showing extremely low oil-adhesion of the underwater superoleophobic surface. Noticeably, different oil wettabilities can be achieved on the resultant surface by controlling the solution pH. Figure S8 and Movie S1 (Supporting Information) show the time sequences of placing an oil droplet onto the resultant surface. In the aqueous solution with pH of 12, the oil droplet is difficult to be placed onto the sample surface because of the ultralow oil-adhesion of the sample surface (Figure S8a, Supporting Information). As the pH value is adjusted to 11, the droplet can adhere to the surface,


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showing higher oil-adhesion (Figure S8b, Supporting Information). On the contrary, in acid aqueous solutions, the oil droplet will quickly spread out once it contacts with the surface, displaying remarkable underwater oleophilicity (Figure S8c, Supporting Information). Furthermore, the femtosecond laser-induced underwater superoleophobic surfaces shows pH-responsive wettability to not only dichloroethane but also a wide range of other oil liquids, such as decane, hexane, chloroform, dodecane, hexadecane, liquid paraffin, petroleum ether, and crude oil. (Figure 5j).

Figure 5. (a-h) Images of an oil droplet on the resultant sample in the aqueous solutions with different pH: (a) pH = 2, (b) pH = 4, (c) pH = 6, (d) pH = 7, (e) pH = 8, (f) pH = 10, (g) pH = 11, and (h) pH = 12. (i) Relationship between the pH of aqueous solutions and oil contact angle (left)/oil sliding angle (right). (j) OCAs of various oils on the resultant copper plates in acidic solution (pH = 2) and alkaline solution (pH = 12), respectively. The insets are the shapes of different oil droplets on the surfaces.

The switchable oil wettability is caused by the combination of the pH-responsive functional groups and the laser-induced micro/nanostructures, as shown in Figure 6. The resultant surface is covered with methyl, ethyl and carboxyl groups. –CH2 and –CH3 are hydrophobic groups with low surface free energy, but –COOH is a strongly hydrophilic group due to the Van der Waals’ force.16 There is a competition between –CH3 and –COOH groups. In the acidic solutions, the carboxylic acid groups are


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protonated, so the methyl groups play a dominant role in wetting states (Figure 6a). Generally, the oil wettability of a sample in water is opposite to its water wettability in air.44,45 Due to the repulsive interaction between water molecules and methyl in acidic aqueous solution, oil droplet is easy to entry into the space between the micro/nanosturctures and water environment (Figure 6c), which can be expressed by underwater Wenzel model:46 ' cos OW =rcosOW

(1)

where θ’OW is the underwater OCA on the laser-induced rough surface, and θOW is the underwater OCA on a flat surface, which is smaller than 90° in acid solution. r is the ratio of the laser-induced actual rough area to the project area, and it is always larger than 1. According to eq 1, θ’OW decreases with the increasing r. Therefore, the acid solution and hierarchical micro/nanostructures endow the surface with underwater quasi-superoleophilicity. When the pH value of the aqueous solution increases to the condition of alkalescence (pH = 8), only part of carboxyl groups are deprotonated due to their incomplete ionization. The ionized carboxyl groups dominate gradually as pH value increases to 10. Bond forms between water molecules and the ionized carboxyl groups (Figure 6b). The resultant surface is easily wetted in alkaline aqueous solutions as the pH value reaches up to 11. When an underwater oil droplet is placed onto the resultant surface, a water layer underneath the oil droplet is trapped on the surface micro/nanostructures, giving rise to superoleophobicity in strong alkaline solutions (Figure 6d). The ratio of the ionized and the unionized carboxyl groups can be adjusted by altering the hydrogen ion concentration in the aqueous solutions so that the reversible transition from underwater quasi-superoleophilicity to superoleophobicity can be easily realized.

Figure 6. Schematic illustration of the pH-responsive wettability between underwater superoleophobicity and quasi-superoleophilicity on the resultant surface.

In the process of femtosecond laser ablation, the laser-irradiated area can be precisely controlled by computer program without expensive masks and complex operation. To suit different application requirements, various patterns can be easily designed and fabricated through selective laser treatment, as shown in Figure 7. The patterned surfaces may present remarkable wetting property in combination with the pH-induced switchable wettability and have potential applications in lab-on-chip


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devices, microfluidic systems, microcreators, and so on.

Figure 7. Various patterns fabricated by selective femtosecond laser ablation.

Conclusions In conclusion, an effective way to fabricate pH-responsive underwater superoleophobic surfaces with switchable wettability by a femtosecond laser is reported. Femtosecond laser processing can endow Cu surface with regular hierarchical micro/nanostructures. Then, the textured surface is covered with the alkyl and carboxylic acids groups by chemical modification. The resultant surface shows superoleophobicity in alkaline aqueous solutions while it shows quasi-superoleophilicity in acidic aqueous solutions. Such a transition between underwater quasi-superoleophilicity (OCA = 12°, pH = 2) and superoleophobicity (OCA = 157°, pH = 12) can be reversibly repeated many times. The switchable wettability results from the variation of the –COOH and –COO- groups in different aqueous solutions. We believe the laser-induced wettability-switchable surfaces will have many important applications, such as detecting the variation of acid/alkaline solutions and controlling oil behavior in water.

Acknowledgments This work is supported by the National Key Research and Development Program of China under the Grant no.2017YFB1104700, the National Science Foundation of China under the Grant nos. 51335008, 61875158, 61435005, and 61805192, the China Postdoctoral Science Foundation under the Grant no. 2016M600786, the


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Collaborative Innovation Center of Suzhou Nano Science and Technology, Instrument Analysis Center of Xi'an Jiaotong University. The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . The relationship between the surface wettability and the laser scanning speed; the profiles of the laser-ablated surfaces; XRD pattern of the laser-irradiated copper surface; SEM images of the resultant surface; XPS spectra of the original laser-ablated surface and the resultant surface with high resolution; the oil wettabilities of the flat and the laser-ablated copper surfaces without modification in different pH solutions (PDF) Process of placing an oil droplet onto the resulatant surface in different pH solutions (Movie S1, AVI)

Author Contributions F.C. directed and supervised the research. J.Y. proposed the main research idea and designed the experiments. J.Z. performed the experiments and wrote the manuscript. Other authors contributed toward significant discussions and revised the paper.

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Repelling or Capturing Bubbles in Water. ACS Appl. Mater. Interfaces. DOI: 10.1021/acsami.8b21465. (46)Wang, S.; Jiang, L. Definition of Superhydrophobic States. Adv. Mater. 2007, 19, 3423-3424.


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figure1 75x34mm (300 x 300 DPI)


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figure3


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figure7 159x179mm (300 x 300 DPI)


Femtosecond Laser-Induced Underwater Superoleophobic Surfaces

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