Robust Super-Amphiphobic Titanium Surface for Liquid/Liquid Mini Separations

Super-amphiphobic surface with low robustness is not suitable for practical application due to its weak mechanical strength. In this work, an in-site growth of micro-/nanoscale flower-like TiO2 on the surface of a titanium mesh was successfully fabricated through hydrothermal synthesis, followed by chemical modification with low-surface-energy heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane. The resultant super-amphiphobic coating was highly repellent to all of the ethanol– water mixtures with surface tensions ranging over 26.0–72.8 mN/m, as well as excellent chemical and mechanical durability. After it was irradiated for 8 h with ultraviolet light, it was used for oil/water and oil/oil mini-separation with the help of its Janus characteristic. This was attributed to its unidirectional penetration for liquid droplets with different surface tension values. This kind of smart super-amphiphobic mesh with photochemical activity could potentially gate and sort liquids via surface tensions.


Introduction
Super-hydrophobic coatings with water contact angles (WCAs) greater than 150 • and water roll-off angles below 10 • have been based on low-surface-energy materials and nano-and microscale roughness [1,2]. However, they are still wetted by organic liquids with low surface tensions [3,4]. Super-amphiphobic surfaces have many important practical applications, including anti-fouling, self-cleaning of traffic signals, corrosion resistance, and membrane separations [5,6]. The control of surface wettability is challenging and requires knowledge in physical laws, chemical compositions, and surface structures [7,8]. Recently, there have been rapid advances in the design and fabrication of super-amphiphobic surfaces [9]. However, the fabrication usually requires complex processes and/or specialized, costly equipment, requiring skilled personnel. Moreover, many fabricated super-amphiphobic coatings have weak repelling ability to the oils with lower surface tension (<35 mN/m), and these fabrication techniques involved are not easily scalable [10,11].
Due to its high catalytic and photo-electrochemical activity, environmentally friendly properties, and low cost, titanium dioxide (TiO 2 ) has become a commonly used highefficiency catalyst. It has been widely used in the preparation of super-hydrophobic materials, photoinduced splitting of water into hydrogen, and the photodegradation of wastewater. There are many techniques for the immobilization of TiO 2 on solid substrates to prepare super-hydrophobic surfaces. Lu created an ethanolic suspension of perfluorosilanecoated titanium dioxide nanoparticles that can be sprayed, dipped into, or extruded onto both hard and soft materials to create a self-cleaning surface that functions even when immersed in oil [12]. Movafaghi et al. utilized tunable, super-amphiphobic, energy-efficient analytical devices with flower-like TiO 2 nanostructures for precisely tailored surface-energy domains to sort droplets via surface tension [13]. Recently, we fabricated a simple U-shaped device with a quasi-super-amphiphobic coating to sort droplets. The surface coating was prepared from an ethanol suspension of beeswax and TiO 2 (25 ± 5 nm) and SiO 2 (7-40 nm) nanoparticles [14]. Generally, the preparation of TiO 2 nanoparticles from solution with different sizes involves a tedious procedure of re-dispersion into certain agents, followed by precipitation, spraying, pasting, or spin-coating onto a substrate.
Here, we report in situ synthesis of TiO 2 nanoparticles on a Ti substrate based on previous reports [15,16]. After hydrothermal etching, followed by chemical modification, a super-amphiphobic titanium mesh was obtained with excellent chemical stability with respect to corrosive liquids, such as acidic, basic, and salt solutions. Compared with previous methods, the prepared meshes exhibited more anti-abrasion, anti-sonication, and anti-peeling robustness. More importantly, the Ti substrate had a switchable wettability after ultraviolet (UV) irradiation, which can greatly broaden applications in liquid/liquid separations because of its unidirectional penetration.

Materials and Methods
A Titanium mesh (99.5% purity, Hebei Kangwei Wire Mesh Co., Ltd., Hengshui, China) with the thickness of 0.60 mm or so and an aperture of 300 mesh was used in our experiments. Fabrication of super-amphiphobic Ti mesh was carried out as follows: Ti mesh (20 mm ×60 mm × 8 mm) was first degreased using acetone and then washed in distilled water. It was then etched in 18% HCl at 85 • C for 15 min to remove the surface oxide layer and thoroughly washed with distilled water. Hydrothermal etching was performed in a closed PTFE-lined stainless-steel autoclave at 160 • C for 2-10 h with a 20 mm hydrofluoric acid solvent [17]. After that, the etched meshes were immersed in 1% ethanol solution of heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane (FOTS) about 1 h for the next application.
Apparatus: The surface morphologies of the pristine and fabricated meshes were characterized by scanning electron microscope (SEM, FEI Company, Hillsboro, OR, USA). The contact angles were determined using a contact angle goniometer (TST-200, Shen Zhen Testing Equipment Co., Ltd., Shen Zhen, China). The rolling angles were measured by advancing or receding 10 ± 0.5 µL droplets on the surface using a micrometer syringe. The rolling angles were measured by tilting the platform until the 10 ± 0.5 µL droplet rolled off from the surface of the mesh. The surface tension of the liquid droplet was measured by the pendent drop method using a CA analyzer. At least three measurements were carried out for the determination of surface tension values of different ethanol-water solutions.

Characterization of Ti Mesh
As shown in Figure 1, no flower-like TiO 2 nanostructures were observed on the pure unetched Ti mesh; however, some appeared on the etched Ti meshes after various times. When the hydrothermal time was increased from 2 h to 10 h, flower-like TiO 2 nanostructures with different particle sizes appeared on the etched Ti meshes. The optimal time of 8 h was chosen to ensure a uniform, flower-like TiO 2 superstructure. In this case, more homogenous 0.5-µm nanoparticles were formed, and a small quantity were aggregated with 2-3 µm diameters. They were then modified with low-surface-energy heptadecafluoro-1,1,2,2tetrahydrodecyl trichlorosilane (FOTS) via immersion. Scanning electron microscope images of the final modified meshes are shown in Figure S1. Relative to the smooth surface of the pure unetched mesh, the hierarchical nanostructures on the surfaces were formed after chemical modification. Although no clear morphology differences between the etched and modified meshes were found, a thin layer of FOTS on the hierarchical surface decreased the surface energy.

Preparation of Super-Amphiphobic Ti Mesh
Figure 2a depicts the procedure for the two-step coating treatment. In the first step, TiO 2 was hydrothermally grown on the Ti substrate to increase surface roughness. An ethanol suspension of FOTS was then applied to the etched mesh via dip coating. Specifically, the etched mesh was immersed in a 1% ethanol solution of FOTS for 1 h. As shown in Figure 2b, the surface wettabilities of the pure unetched mesh, etched mesh, and etched mesh after chemical modification were compared. Both the uncoated and TiO 2 -grown meshes were wetted by water. However, the final modified mesh exhibited spheres of both water and soybean oil, indicating that the surface was super-hydrophobic and highly omniphobic. These results are in good agreement with the Cassie and Baxter model [13,14]. The re-entrant geometries allowed air to be trapped in the crevices and gaps of the micro-/ nanostructures under the liquid droplets, resulting in the super-amphiphobic property.

Fabrication of Unidirectional Oil-Transport Ti Mesh
To prepare a unidirectional oil-transport mesh, we used strong UV light to irradiate the modified mesh on one side [6,18]. This irradiated side of the super-amphiphobic surface was transformed in various degrees to hydrophilic and omniphilic surfaces after different UV exposures. Longer UV exposures led to super-hydrophilicity and super-omniphilicity because of the photochemical activity. Drops (10 µL) of a series of ethanol-water mixtures (0%, 20%, 40%, 60%, and 80% ethanol, v/v) with different surface tensions on the modified mesh are shown in Figure 3a (also see Table S1 in Supporting Information) [13]. Without UV irradiation, the super-amphiphobic surface was highly repellent to all of the ethanol-water mixtures, with surface tensions ranging over 26.0-72.8 mN/m. All of the liquid droplets had a marble appearance on the mesh, with contact angles (CAs) in the range of 120-163 • . There were no differences in CAs between the two mesh sides (see Figure 3a). For the modified mesh after 4 h of UV irradiation, the 80% ethanol-water droplets (surface tension of 26.0 mN/m) exhibited unidirectional transport, whereas the mesh prevented the other ethanol-water mixtures (0-60%) from penetrating both sides in Figure 3b. After 6 h of irradiation, the CAs for all the liquid droplets decreased on the unirradiated sides (back) in Figure 3c. For the mesh after 8 h of irradiation, directional transport for 60% ethanol-water occurred from the unirradiated to the irradiated face (see Figure 3d). The unirradiated side (back) remained super-hydrophobic after 4-8 h of UV irradiation on the front side, and the irradiated side became hydrophilic and super-oleophilic after 4 h of irradiation. Thus, the front and back sides exhibited different surface wettabilities, which could be used for unidirectional transport of liquid droplets with different surface tensions.

Self-Cleaning, Chemical, and Mechanical Stability of Super-Amphiphobic Ti Mesh
Self-cleaning dirt removal tests were performed on super-amphiphobic surfaces that were slightly tilted. As shown in Figure S2, a droplet from a syringe removed dirt (Suan red III powders) and left nothing on the surface. Usually, a super-amphiphobic coating with a micro-or nanoscale surface roughness is mechanically weak and readily abraded [19,20]. However, the fabricated hard titanium substrate was mechanically robust. An ultrasonic cleaner was used to investigate its mechanical durability, as shown in Figure 4a. After ultrasonication for 60 min, the super-amphiphobic mesh still remained super-hydrophobic with WCAs greater than 150 • , indicating that the micro/nano structure was not stripped off by the strong vibrations. The mechanical durability was also evaluated by tape-peeling tests, as demonstrated in Figure 4b. The super-hydrophobicity was still retained after 60 on/off peeling cycles, and the WCAs remained in the range of 155-163 • . The sandpaper abrasion tests were carried out as illustrated in Figure 4c. The super-amphiphobic Ti mesh weighing 100 g was placed face down to sandpaper and then moved for 10 cm along the ruler. The static WCAs were still 156-165 • over 60 cycles, indicating that the surface coating was not destroyed by mechanical abrasion. The chemical stability was tested in strong acid, base, and saline solutions. As shown in Figure 4d, after the super-amphiphobic meshes were immersed in 1-mol/L HCl, 1-mol/L NaOH, and 3.5-wt% NaCl solution for 24 h, the corresponding WCAs were 154 • , 157 • , and 158 • , respectively.

Liquid/Liquid Mini-Separations of Ti Mesh
The TiO 2 exhibited crystallinity and a homogeneous topography with particle sizes in the range of 0.5-2.5 µm. As discussed above, the irradiated side was transformed to hydrophilic and various omniphilic degrees after various UV exposure times. A Ti mesh after irradiation for 8 h with a 254 nm UV bench lamp was used to gate liquids with different surface tensions. As shown in Figure 5a, a water droplet was blocked on the modified mesh surface with its UV-irradiated face downward, whereas a droplet of soybean oil (dyed with red Sudan) penetrated the mesh and blotted the paper below it. This demonstrated a mini-separation of a water/oil mixture [21,22]. More importantly, it was also used for oil/oil separations of two immiscible organic compounds. Specifically, a formamide droplet (57.5 mN/m) was blocked on the surface, while a soybean oil droplet (32.3 mM/m) penetrated the mesh (see Figure 5b). The formed TiO 2 on the surface of Ti mesh after thermal synthesis is photosensitive. When TiO 2 is irradiated by UV light, pairs of electron holes are created on its surface, which can react with lattice oxygen to form surface oxygen vacancies. Water molecules can be kinetically directed to these vacancies and later dissociated to generate two hydroxyl groups at the site of each defect. Then, more monolayers and multilayers of water are formed by molecular adsorption, which leads to the formation of hydrophilic domains on the TiO 2 surface. Despite the change of the surface chemistry on the "front" surface, the "back" side, which did not receive direct UV irradiation, exhibits similar surface features with those of the unirradiated substrate. TiO 2 has strong absorption to UV light, and its coating on the mesh blocks UV light penetration. This would lead to variation in the surface chemistry with depth after UV irradiation, and a gradient of hydrophilicity-to-super-hydrophobicity and an oleophilicity-to-oleophobicity along the mesh thickness would form from the UV-irradiated "front" to "back" surfaces. The oleophilicity-to-oleophobicity gradient profile could be well tailored by the UV irradiation time, which led to the selectivity of the unidirectional liquid transport property [23][24][25][26][27]. The results demonstrated that the super-amphiphobic Ti mesh with one side irradiated by UV at different time is an important engineering material that could be potentially used for large-scale separations of liquid/liquid with different surface tensions.

Conclusions
In summary, in situ grown, flower-like TiO 2 nanostructures were fabricated via a hydrothermal process on a Ti mesh surface in the presence of HF. After further modification with a low-surface-energy coating, a super-amphiphobic material was obtained that exhibited self-cleaning, as well as excellent chemical and mechanical stabilities. One side of the mesh was then exposed to UV irradiation. Because of the high photochemical activity of the Ti substrate, the two sides of the modified Ti mesh then exhibited different surface wettabilities and directional transport for liquid droplets having various surface tensions. It was thus used to demonstrate mini-separations of oil/water and oil/oil mixtures, with a potential to sort liquid droplets with different surface tensions.