Photocatalytically Active Superhydrophilic/Superoleophobic Coating.

Hydrophilic materials are easily fouled by organic contaminants owing to their high surface energy, and this oil-fouling problem severely hinders their use in practical applications. To address this challenge, herein, a hydrophilic coating with oil repellency and photocatalytic activity is developed by a spray-casting process. In the air surrounding, a water droplet spreads over the coating surface completely, while oil droplets exhibit contact angles more than 150° and moving on the coating freely. The water-wetted coating still had oil repellency, as the water layer on the coating surface can act as a lubricant to repel oil. Although methylene blue aqueous solution contaminates the coating by wetting it completely, these water-soluble organic molecules can be removed by UV illumination, due to the photocatalytic activity of the coating. Exploiting its water-attracting and oil-repelling properties, the coating deposited on a copper mesh is applied as a multiplatform for oil-water separation with high separation efficiency. This study provides a novel and efficient way to solve the oil-fouling problem of hydrophilic materials.

Introduction

Superhydrophilic surfaces induce the complete spreading of water, leading to water contact angles of less than 5°.[1,2] This extreme water-loving behavior makes such surfaces favorable for a broad range of research and commercial applications including water harvesting, self-cleaning, antifogging, antireflection, enhancement of boiling heat transfer, and biomedical applications.[3−10] Until now, various superhydrophilic surfaces have been developed by mimicking the biological surfaces in different ways.[11−15] However, due to their intrinsic oleophilicity, traditional superhydrophilic surfaces are readily fouled by oils that are hard to remove. This susceptibility to oil fouling severely hindered the use of superhydrophilic surfaces in practical applications. To address this challenge, many groups have tried to develop superoleophobic surfaces by creating specific structures on surfaces.[16−22] For example, Tuteja et al. created re-entrant structures on electrospun polyhedral oligomeric silsesquioxane fiber (POSS) surfaces and meshes.[16] Kim and Liu developed doubly re-entrant textures to realize superoleophobic properties without low surface energy coating.[19] Seeger et al. fabricated superoleophobic coatings by producing multiscale surface roughness.[21] Although the obtained surfaces had oil repellency, they lose their originally superhydrophilic property at the same time. It is highly desirable to develop a surface with superoleophobicity while retaining its superhydrophilicity. However, engineering surfaces exhibiting both wetting properties is hard to achieve based on surface tension theory, as it is requiring a complex interface that simultaneously has a surface energy higher than that of water and lower than that of oil.[16,23−25] Recently, Yang et al. fabricated an in-air superhydrophilic/superoleophobic nanocomposite film through spray coating of silica nanoparticle/poly(diallyldimethylammonium chloride)-perfluorooctanoate solutions.[26] Pan et al. developed nanoparticles and hydrophilic-unit-induced short-chain fluorinated compounds as basic constituents to fabricate superhydrophilic/superoleophobic surfaces.[27] Other approaches have included UV-light and ammonia-triggered transitions to achieve the superhydrophilic/superoleophobic wetting behavior.[28−30] For these superhydrophilic/superoleophobic surfaces, they repel most oils while retaining their superhydrophilicity; however, organic molecules dissolved in water can penetrate the surface texture and thus contaminate the surfaces. Thus, additional research should be carried out to address this challenge.

Titanium dioxide (TiO2) is a widely used metal oxide photocatalyst in our daily life, and it generates electron–hole pairs under light irradiation, causing oxidation or decomposition of most organic molecules.[31,32] This photocatalytic activity is benefiting for antioil fouling and self-cleaning. By creating a superhydrophilic surface with in-air superoleophobicity and photocatalytic activity simultaneously, we are able to solve the oil-fouling problem for superhydrophilic materials perfectly. The oil repellency is expected to clean oil liquids, while the photocatalytic activity is hoped to decompose the water-soluble organic molecules. In this study, we developed a superhydrophilic coating with superoleophobic property and photocatalytic activity through spray casting of a hybrid solution containing hydrophilic components, oleophobic components, and TiO2 nanoparticles. In air conditions, water droplets wet the coating completely, while oil droplets with contact angles more than 150° were rolling off it readily without any penetration. The water-wetted coating still displayed oil repellency by establishing a slippery state. The water-soluble organic contaminant (i.e., methylene blue) on the coating surface or in water can be removed directly and completely by UV illumination, owing to its photocatalytic activity. Exploiting its water-attracting and oil-repelling properties, the coating sprayed on a copper mesh was used as a multiplatform for oil–water separation. This study is hoped to provide a simple but efficient way to solve the oil-fouling problem of superhydrophilic materials.

Experimental Section

Materials

Perfluorooctanoic acid (PFOA), TiO2 nanoparticles with an average size of less than 25 nm, bis(3-(trimethoxysilyl)propyl)amine (BTMEPA), and (3-aminopropyl)triethoxysilane (APTES) were all purchased from Sigma-Aldrich. Sodium hydroxide (NaOH) and ethanol were provided by Sinopharm Chemical Reagent Company. Commercially available glass slide and copper mesh were cleaned with acetone and deionized water sequentially in an ultrasonic cleaner before use.

Coating Fabrication

A total of 0.1 mL of BTMEPA and 0.1 mL of APTES were added into 15 mL of ethanol under magnetic stirring to form a homogeneous solution (denoted solution A). Then, 0.5 g of PFOA and 0.05 g of NaOH were added into 15 mL of ethanol under stirring, and the solution was kept stirring for 2 h, to synthesize sodium perfluororooctanoate (denoted PFOA-Na). Then, 3 g of TiO2 nanoparticles was ultrasonically dispersed in PFOA-Na ethanol solution, and this solution was denoted solution B. Then, solution A was added dropwise into solution B under stirring, and the hybrid mixture was kept stirring for 3 h. The hybrid mixture was sprayed onto slide glass and copper mesh using a spray gun connected to a compressed nitrogen source (0.2 MPa). The distance between the spray gun and the substrate and the spraying time is 20 cm and 1 min, respectively. Finally, the sprayed coatings were dried in an oven at 60 °C for 1 h.

Characterization

Scanning electron microscopy measurements were carried out using a JSM-6701F field-emission scanning electron microscope (FESEM, JEOL, Japan). Energy-dispersive X-ray spectroscopy (EDX) analysis was conducted on a NORAN System Six X-ray microanalysis system (THERMO) attached to the FESEM. Contact angle (CA) and sliding angle (SA) measurements were performed using Krüss DSA 100 (Krüss Company, Ltd., Germany) apparatus at ambient temperature. The volume of probing liquids in the measurements was 5 μL. The average CA and SA values were determined by measuring the same sample at five different positions. The optical images were obtained by a digital camera (Nikon).

Results and Discussion

Surface Texture and Composition Analysis

The resulting coating was developed through a spray-casting process. The spray deposition method provided coatings with hierarchically rough morphologies on substrates. As shown in Figure a, the coating surface is covered by tons of microaggregates that can be attributed to the spray coating process. Moreover, Figure b shows that the microagglomerates are superimposed with nanosized particles, endowing the coating surface with a hierarchical structure. Energy-dispersive X-ray spectroscopy (EDX) analysis demonstrated that the coating surface was primarily composed of O, F, Ti, C, N, Na, and Si, as shown in the Figure b inset. The N and Na in the energy-dispersive system (EDS) originates from the aminosilane (namely BTMEPA and APTES) and PFOA-Na, respectively. The fluorine content of the coating calculated from EDX was as high as 10.29%, which is indicative of surface enrichment of low surface energy fluorocarbon groups.

Figure 1

SEM images of the resulting coating at low (a) and high (b) magnifications; the inset in (b) shows the EDX spectrum of the resulting coating.

Wetting Behavior

The created coating displayed superoleophobic wetting property in air. Oil droplets including rapeseed oil, hexadecane, prude oil, and engine oil were displaying spherical shape on the coating surface (see Figure a) and were able to roll off the coating easily when the surface was tilted slightly (see Video S1). The bright, reflective surface visible underneath the probing oils indicated that an air layer was trapped between the droplets and surface texture, and thus a composite solid–liquid–air interface was established.[33] Other oil droplets such as dichloromethane, toluene, and dodecane all had a CA value greater than 150° and an SA value less than 10° (see Table S1). When submerged into the engine oil, a mirrorlike phenomenon was observed on the coating surface (see Figure b), owing to the reflectance of light at the air layer trapped on the coating surface. The coating kept its surface dry after one day of immersion in engine oil, indicating its robustness against pressure-induced wetting. Interestingly, this superoleophobic coating also exhibited an unusual superhydrophilic wetting behavior. When contacting the coating surface, the water droplet was spreading on it quickly and completely, as shown in Figure c and Video S2, and the remaining water was forming a water layer on the coating surface. This water spreading time was relatively fast when compared to some materials reported, particularly considering that no additional treatments were required.[26,28,29]

Figure 2

(a) Droplets of water and different probing oils on the coating; (b) mirrorlike phenomenon was observed on the coating surface when submerged in oil; (c) contact angle profiles of a water droplet when contacting the coating surface; (d) droplets of different oils on the water-wetted coating; (e) engine oil droplet was moving on the water-wetted coating freely.

Moreover, this water-wetted coating still displayed oil repellency. Due to the good compatibility of the coating and water, the water layer on the coating surface can act as a lubricant to repel oil droplets, and a thus slippery state was achieved on this water-wetted coating, as shown in Figure d. Oil droplets were moving on the water-wetted coating easily, due to the mobility of the water lubricant. For example, the engine oil droplet was sliding on the coating surface with a speed of about 1.19 cm/s, as shown in Figure e. Importantly, the water-wetted coating was returning to its superoleophobic state after removing the water by heating (see Video S3), and thus a reversible wetting transition between the slippery state and the superoleophobic state was achieved by water wetting and heating treatment alternately, as shown in Figure S1.

Mechanism of the Superhydrophilic/Superoleophobic Wetting Property

The total free energy of a solid (γs) is considered as the sum of contributions from dipole-hydrogen bonding (γsd) and dispersion forces component (γsp), as shown in eq .[34,35]According to Owens and Wendt’s theory (ref (34)), the solid surface free energy can be estimated as follows:where γliquid is the surface tension of the test liquid, and θ is the liquid intrinsic contact angle on the solid surface.

As for liquids with high polarity values such as water, both dipole-hydrogen bonding and dispersion forces contribute to the surface tension. Thus, the water intrinsic contact angle (θw) can be expressed asGiven that the polar contribution of the nonpolar or low nonpolar oils is negligible in the surface tension component,[36] the oil intrinsic contact angle (θo) can be expressed aswhere γw and γo in eqs and 4 are the surface tension of water and oil, respectively.

Based on eqs and 4, it can be concluded that a smaller γsd value is needed for a bigger θ value, while a bigger γsp value is required for a smaller θw value. Thus, combining superhydrophilicity and superoleophobicity in one surface can be realized if the surface has a sufficiently large γsp and a sufficiently small γsd simultaneously.[37]

As for our coating, the fluorinated groups (namely −CF3 and −CF2−) minimize the γsd constituent, resulting in high resistance against oils such as n-hexadecane, toluene, and rapeseed oil. While the hydrophilic components (namely carboxyl, quaternary ammonium groups, and sodium ions) increase the γsp constituent, leading to a strong affinity to polar water molecules. This hydrophilic/oleophobic wetting behavior of the coating was magnified when the surface roughness was increased by adding TiO2 nanoparticles into it. As a result, the resulting coating exhibited superhydrophilic and superoleophobic wetting properties simultaneously in air, as shown in Figure .

Figure 3

Schematic diagram illustrating the superhydrophilic and superoleophobic wetting behavior of the coating.

Stability of the Coating

We next studied the stability of the resulting coating. To study its mechanical stability, a series of manual tests were conducted, as shown in Figure a–e. We tested the resistance of the coating toward bending. The coating did not alter and delaminate after 100 hand-bending cycles, and rapeseed oil moved freely on the bent coating surface, as shown in Figure a,e. Also, rapeseed oil still displayed a spherical shape on the finger-pressed coating surface with the CA value remaining unchanged (see Figure b,f). Similarly, the coating kept its superhydrophilic/superoleophobic wetting properties after other tests including tape peeling, water jetting (water pressure: 0.07 Mpa, flushing time: 1 h), and thermal treatment (see Figure c,d,g,h and Table S2). The superhydrophilic/superoleophobic coating also had long-term stability, as evidenced by the outdoor exposing test. The coating samples displayed a static contact angle of 0° for water and 156° for hexadecane after placing in an outdoor place for 60 d (April and May 2019 in Yantai, China).

Figure 4

Manual tests including hand bending (a, e), finger pressing (b), tape peeling (c), and water jetting (d) for evaluating the robustness of the coating. The coating kept its superoleophobic property with an oil contact angle larger than 150° (f–h) after the relevant tests.

Photocatalytic Activity Analysis

To investigate its photocatalytic activity, the coating was contaminated by a water-soluble dye (namely methylene blue). Methylene blue aqueous solution (3.54 × 10–5 mol/L, 2.7 mL) was spreading across the coating, and this dye was retained on the coating when water was removed by evaporation. However, this dye contamination was decomposed by exposure to UV illumination. Herein, we used a UV lamp (wavelength: 320–370 nm, intensity: 5 mW/cm2) with a distance of 10 cm to the sample for the illumination treatment. As shown in Figure a, the color of the dye-contaminated coating changed from dark blue to white with the UV illumination time going on. The photocatalytic decomposition of organic molecules was not limited to methylene blue, and other common containments such as methyl orange can also be decomposed by our coating under UV illumination. As a result, our coating demonstrated an enhanced self-cleaning property for both oil liquids and organic molecules dissolved in water, given its combination of oil repellency and photocatalytic activity.

Figure 5

(a) Color changes of methylene blue–contaminated coating as a function of irradiation time; (b) UV–vis spectrum of methylene blue dye aqueous solution. The solution was illuminated by a UV lamp in the presence of the coating. The insets show the color changes of the solution as a function of irradiation time.

It was found that the surface structure was retained, while the content of F element was reduced from 10.29 to 6.47%, after UV irradiation. As a result, the surface was losing its superoleophobic wetting property with the contact angle varying from 157 to 114° for engine oil. However, the UV-irradiated surface was still displaying superhydrophilicity, and it was having slippery oil-repellent property when wetted by water. An engineering oil droplet was moving freely on the water-wetted surface, as shown in Figure S2b.

By further utilizing its photocatalytic activity, our coating was applied as a catalyst for water purification. Methylene blue was dissolved in water (3.54 × 10–5 mol/L, 40 mL) and used as a contaminant. As shown in Figure b, the concentration of methylene blue in the water was decreasing gradually with increasing the illumination time, as evidenced by the UV–vis spectrum. After being illuminated by a UV lamp, all of the methylene blue dye in water were decomposed completely, leaving clean water eventually (see Figure b insets).

TiO2 is one kind of photocatalysts with photocatalytic activity, and it is activated to generate electron–hole pairs under light illumination. The electron–hole pairs can oxide or decompose most organic molecules to achieve self-cleaning.[31,32] As for our coating, TiO2 nanoparticles are partially exposed and allow for photocatalytic activity. Methylene blue molecules dissolved in water can be decomposed by the photocatalytic activity of TiO2 under UV illumination, when contacted with our coating. Possible applications of our photocatalytically active coating can be envisioned in the fields of energy conservation, water purification, and containment decomposition.

Oil–Water Separation

Oil–water separation is a worldwide challenge and has recently generated intensive interest.[38−40] When our superhydrophilic/superoleophobic coating was deposited on a copper mesh, it can be applied as a multiplatform for oil/water separation. The superhydrophilic/superoleophobic copper mesh was used for the first time as a separation membrane for oil/water separation. As shown in Figure a, when the oil/water mixture was poured onto the copper mesh surface, water penetrated through the mesh and flowed down the beaker underneath, while oil (toluene) remained on the mesh surface. Such a design lowered the propensity of the surface being fouled by the oil phase and enabled a gravity-driven separation process. The water permeating flux of the separation membrane is as high as 2750 L/(m2 h bar). Figure b shows the process of collecting oil from water using the copper mesh to construct a spoon device. When the spoon device was brought into contact with the oil/water mixture, water wetted the spoon surface completely and oil was floating onto the spoon surface. After taking out, oil was collected onto the spoon device, and there was no visible oil left in water. The copper mesh was then applied as a water skimmer for water collection through mounting it on the open end of a glass beaker. As shown in Figure c, the floating water was passed through the partly submerged mesh and flowed down along the glass wall of the beaker, while oil was blocking outside the collection vessel. Importantly, as the collected water was stored in the bottom vessel, the water collection capacity was depending on the volume of the vessel rather than the copper mesh, allowing its water collection capacity to be unlimited theoretically. Herein, we should emphasize that this water skimmer is completely different from the conventional skimmer devices based on removing oils from bulk water, and it has been seldom reported.

Figure 6

Superhydrophilic/superoleophobic coating-deposited copper mesh was applied as a separation membrane (a), spoon strainer (b), and water skimmer (c) for oil–water separation.

Importanly, the oil–water separation efficiencies for the separation membrane, water skimmer, and spoon strainer devices were all more than 99% and they were retaining their high separation efficiency even after 20 cyles of oil–water separation, as shown in Figure d. Also, the water permeating flux of the separation membrane varied slightly after 20 cyles oil–water separation, as shown in Figure S3a. Due to its superhydrophilic/superoleophobic wetting properties, the water-wetted copper mesh displayed a low affinity to oils after oil–water separation (see Figure S3b), and thus it prevented the pores of the mesh to be fouled or blocked by oils. This antioil fouling property allowed the superhydrophilic/superoleophobic copper mesh to retain its high oil–water separation efficiency. These results indicated that our superhydrophilic/superoleophobic coating deposited on a piece of copper mesh is a promising candidate for membrane separation industries.

Conclusions

We have developed a superhydrophilic coating with oil repellency and photocatalytic activity by a spray-casting process. In the air surrounding, water wetted the coating completely without additional treatment, while oil droplets with contact angles bigger than 150° were able to move on the coating freely. This water-attracting and oil-repelling wetting properties were due to the unique surface chemistry of the coating that has a smaller dipole-hydrogen bonding and a higher dispersion forces component. Moreover, the water-wetted coating still had oil repellency by establishing a slippery state. Water-soluble methylene blue dye on the coating can be decomposed by the photocatalytic activity of TiO2 in the coating under UV illumination. The photocatalytic activity of the coating was also demonstrated by the purification of methylene blue–contaminated water under UV irradiation. Exploiting its water-attracting and oil-repelling properties, the coating deposited on a copper mesh was applied as a multiplatform for oil–water separation. Combining oil repellency and photocatalytic activity provided a novel and efficient way to solve the oil-fouling problem for (super)hydrophilic materials. We anticipate that the novel wetting properties, photocatalytic activity, oil–water separation efficiency, and fabrication simplicity of our coating may lead to broad applications such as self-cleaning, water purification, and oil spill treatment.