A Brief Review on Bioinspired ZnO Superhydrophobic Surfaces: Theory, Synthesis, and Applications

Due to the intensive demand in the development of superhydrophobic surfaces both in theory and application, superhydrophobic research on ZnOmaterials has exponentially grown over the last several years. One of the main advantages of the ZnOmaterials is the relative easiness to produce various surface morphologies, which is quite meaningful to study the influence of morphology on wetting property.The synthesis approaches of ZnOmaterials include thermal oxidization, hydrothermalmethod, chemical etching, spray coating technique, electrochemical method, and others. This review is a representation of the recent achievement on ZnO superhydrophobic surfaces.


Introduction
When a liquid drop is in contact with a solid surface, two distinct regimes of liquid spreading are observed depending on the spreading factor  ( =  sg − ls −), where  ls ,  sg , and  are the interfacial tensions between liquid and solid, solid and gas, and gas and liquid [1].The spreading factor  indicates whether the formation of a liquid film is energetically favorable on a solid surface.A positive spreading factor  means that the energy state with a liquid film on the solid surface is lower than that without a liquid film, implying that a liquid film will be preferably formed on the solid surface, that is, complete wetting.A negative spreading factor , on the other hand, means a liquid drop will be formed on the solid surface with a certain contact angle, that is, partial wetting.In the partial wetting regime, if a contact angle is less than 90 ∘ , a surface is called hydrophilic.If a contact angle is larger than 90 ∘ , a surface is called hydrophobic.In particular, a surface with a contact angle larger than 150 ∘ is considered to be superhydrophobic [2].Therefore, the surface wetting property, that is, droplet contact angle, is closely related to the interfacial energies.However, as there exist irregular or regular patterns on the liquid and solid interface on superhydrophobic surfaces, droplet contact angle should be determined by overall considering interfacial energies and the structural parameters.
Meanwhile, superhydrophobic surfaces have kept attracting considerable interests due to the observation of various superhydrophobic phenomena in nature [2][3][4][5][6][7][8][9][10][11][12][13].The most representative example of natural superhydrophobic surfaces is a lotus leaf [2].On a lotus leaf, microscale bumps are covered with wax crystalloids.These surface structures render a liquid in contact with only the top of the surface structures, thereby resulting in the significant decrease of the adhesion energy between a liquid and the solid surface.This feature enables a lotus leaf to possess water repellency and selfcleaning properties.Then, several other examples of natural superhydrophobic surfaces have been discovered, including wings of butterfly [4], duck feathers [6], legs of the water strider [7,8], integument of water walking arthropod [11], and covering of the desert beetle [12,13].Different structural morphologies on these surfaces result in the superhydrophobicity, which helps to realize the decontamination, drag reduction, or even water collection.Therefore, both the scientific world and engineers show great interests in such special surfaces.In one case, it is highly desirable to make clear how the interfacial energy influences the wetting property on patterned surfaces.In another case, it is also quite important to design different structures to achieve desired superhydrophobicities.
The wetting property, that is, contact angle, can be calculated by considering the force balance at the three-phase contact line.Alternatively, the contact angle at which the thermodynamic energy becomes minimum can be sought.The detailed calculation methods will be introduced in the following section.Meanwhile, as various synthesis techniques to engineer superhydrophobic surfaces have already been developed and reviewed in several articles [14][15][16][17], for example, sol-gel reactions [18][19][20], electrochemical deposition [21][22][23], layer-by-layer deposition [24][25][26][27], spin coating [28], plasma technology [29], and electrical spinning [30], and the materials utilized in these approaches vary from polymers, metals, and other inorganic materials to composites [31][32][33][34][35][36][37][38], we will not cover all these well-known experimental methods and materials in this brief review.Alternatively, we will focus on the recent advances in ZnO superhydrophobic materials, which feature a direct wide band gap (3.37 eV) and large exciton binding energy (60 meV), which have been promising for optoelectronic devices and functional materials such as solar cells [39], light emitting diodes [40], sensors [41], and photodetector [42].It is expected to explore multifunctional superhydrophobic surfaces based on ZnO [43][44][45].
In this review, therefore, the scope has been limited to the description of wetting property of superhydrophobic surfaces and also the recent advances in synthesis approaches of different ZnO superhydrophobic materials.There are three sections, including theory in wetting property, synthesis approaches, and conclusions.

Theory
2.1.Apparent Contact Angle.The contact angle is the manifestation of the interaction among interfacial tensions of the liquid-gas interface , liquid-solid interface  ls , and solidgas interface  sg and can be calculated by considering the force balance at the three-phase contact line.It is assumed that interfacial tensions along the horizontal direction at the three-phase contact line are at equilibrium, thereby resulting in the following Young relation (Figure 1(a)): Alternatively, the contact angle at which the thermodynamic energy becomes minimum can also be sought.When a three-phase line makes a virtual displacement of  as shown in Figure 1(b), the accompanying change of thermodynamic energy is calculated in the following way [46]: At thermodynamic energy equilibrium,  should be zero, and this condition leads to Young's relation as well.
The above analysis is based on the assumption that the solid has a flat surface.However, the thermodynamic approach can also be extended to a superhydrophobic surface as long as the length scale of the roughness is much smaller than the size of the liquid drop.On a rough surface, two distinct configurations of a liquid drop can be envisioned.First, it can be assumed that the liquid completely wets the rough surface while it moves by .Then, a liquid-solid interface will displace a solid-vapor interface by  as shown in Figure 2(a) instead of  on a flat surface, where roughness  is defined as the ratio of actual surface area versus projected surface area ( > 1).Then, the accompanying change of thermodynamic energy will be expressed as follows: where   is an apparent contact angle on a rough surface.The condition for thermodynamic energy minimum leads to the following Wenzel equation [47]: As indicated by this equation, the Wenzel model predicts that roughness amplifies a surface wettability.That is, roughness makes a hydrophilic surface more hydrophilic and a hydrophobic surface more hydrophobic.
In another case, it can also be assumed that a liquid does not penetrate into the roughness, contacting only the top surfaces of the roughness.In this case, a composite interface of liquid-gas and liquid-solid between the overall liquid and solid interface is resultant as shown in Figure 2(b).Then, the corresponding energy change will be given by where   < 1 is an area fraction of liquid-solid interface and   is an apparent contact angle on a rough surface.The condition for the minimization of the thermodynamic energy leads to the following Cassie-Baxter equation [48]: Different from the Wenzel model, the Cassie-Baxter model predicts that a surface becomes more hydrophobic as the solid fraction gets smaller regardless of the intrinsic contact angle on the surface.

Contact Angle Hysteresis.
On any real solid surface, people found that there exists a wide range of "metastable" contact angles when a liquid meniscus scans the solid surface.Because there are free energy barriers which exist between these metastable states, a true "equilibrium" contact angle is almost impossible to measure in real time.The previously mentioned Wenzel and Cassie theories are therefore only applicable in prediction of the thermodynamically stable contact angles in theory.Therefore, contact angle hysteresis defined as the difference of advancing angle and receding angle is usually measured to fully characterize any surface.A typical method to obtain an advancing contact angle is to gradually inject liquid into a sessile drop with a syringe and measure the contact angle when the liquid-solid-air contact line subsequently starts to move outward.Conversely, the method to obtain a receding contact angle is to gradually withdraw liquid out of a drop and measure the angle when the contact line starts to move inward.In another case, tilting angle   can be adopted to quantify this hysteresis effect.For example, when a liquid drop is placed on a titled plate as shown in Figure 3, a drop will not slip off until the following condition is satisfied [49]: where  is a geometric factor that depends on the structure types, and determines the effective three-phase contact line length along the droplet boundary.The surface that has stronger pinning force will exhibit larger   value, according to which the pinning force can be calculated.
As the apparent contact angle is modified on a rough surface from the intrinsic angle on a smooth surface, a contact angle hysteresis is also modified.While both the Cassie-Baxter model and Wenzel model predict the increase of contact angle on a rough hydrophobic surface, they predict opposite trends for a contact angle hysteresis.It is widely accepted that a nonwetted (i.e., Cassie-Baxter state) surface exhibits a decrease of contact angle hysteresis due to the reduced contact area between a liquid and a solid, and a wetted surface (i.e., Wenzel state) shows a significant increase due to the increased contact area between liquid and solid.

ZnO Superhydrophobic Surfaces
According to the theoretical analysis, the wetting states and the closely related wetting behavior vary with the surface structures and surface energies.Meanwhile, great progress has been achieved in the design and synthesis of bioinspired superhydrophobic surfaces based on different ZnO structures , among which most of them were modified with low surface energy materials, for example, silane and fluorination, to eliminate the influence of polarity of ZnO materials on wetting property.We reviewed the related literature published since the year of 2010.Here are the main synthesis methods introduced.

Thermal Oxidization.
In microfabrication field, thermal oxidation is an effective way to produce a thin layer of oxide on the surface of a material.This technique forces an oxidizing agent to diffuse into the material at high temperature and react with it.The materials can be processed in this technique include silicon, metal, and others.By using thermal oxidization method, researchers got ZnO superhydrophobic surfaces [50,51].Barshilia et al. deposited ZnO coatings with varying thicknesses (300-1800 nm) by sputtering a Zn target in Ar plasma followed by 1 h oxidation at 350 ∘ C in O 2 environment [50].ZnO coatings prepared under these conditions were further annealed for 2 h at 450 ∘ C in vacuum to improve the optical transparency.Except from the extraordinary water repellency (with contact angle > 155 ∘ ) as indicated in Figure 4, they even proved that ZnO superhydrophobic coating on the absorber surface has improved absorptance (>0.96) and excellent broadband antireflection in the visible range of the solar spectrum.The multifunctional ZnO coating was stable up to 450 ∘ C (in air and vacuum), indicating its reliability for high temperature photothermal conversion applications.

Hydrothermal Method.
Hydrothermal method is a useful tool to produce different chemical compounds and materials using closed-system physical and chemical processes flowing in aqueous solutions at high temperatures and certain pressures, and is also widely adopted to synthesize different ZnO micro/nanostructures [52][53][54][55][56][57][58][59][60][61][62].Mondal et al. fabricated nanocolumnar thin film (Figure 5) under modified hydrothermal conditions [52].The growth of ZnO nanocolumnar on glass surfaces takes place as a result of the hydrolysis of zinc sulfate by ethanolamine at 100 ∘ C without employing any templates or surfactants.The evolved superhydrophobic thin film becomes hydrophilic upon UV light exposure and the film reverts back to its original superhydrophobic character upon storage in the dark.Due  to UV light illumination an electron-hole pair is generated in the ZnO thin film.Some of the holes react with lattice oxygen leading to oxygen vacancies in the film.The photogenerated electrons are captured by lattice zinc ions to form surface trapped electron sites.Hydroxyl adsorption on the photogenerated defect sites is kinetically favorable.This leads to the dissociative adsorption of water molecules at these sites, inducing hydrophilicity.However, on dark storage the superhydrophilic thin film recovers its original superhydrophobicity.This is because exposure of the surface to air leads to an increase of the thermodynamically preferred oxygen adsorption and the replacement of hydroxyl groups adsorbed on the defect sites by oxygen atoms gradually takes place.
Wu et al. synthesized ZnO structures with three scales of roughness on stainless steel mesh film by a simple hydrothermal method [53].After being modified with a low surface energy material, for example, Teflon, these films exhibit superhydrophobic and superoleophilic properties.They also demonstrated that the unique properties of the as-prepared films match well with the requirements for the effective separation of oil and water mixtures (Figure 6).

Chemical Etching.
Chemical etching is a process of dissolving metals or semiconductors in acids to make them into a particular shape.It has also attracted many interests in the synthesis of ZnO superhydrophobic surfaces.Hou et al. fabricated ZnO submicrorod films (Figure 7) on zinc sheets through an H 2 O 2 -assisted surface etching process and subsequent surface modification with a monolayer of 1H,1H,2H,2H-perfluorodecyltriethoxysilane [63].

Spray Coating
Technique.ZnO superhydrophobic surfaces have also been fabricated by a simple and cost-effective spray coating technique [64][65][66][67][68]. Wu et al. produced ZnO superhydrophobic surfaces based on microscale coffee-ring patterns by this technique [64].The ZnO crystal cells generated by the degradation of zinc acetate solution at high temperature accumulate at the wetting circle of microdroplet and assemble into a coffee-ring pattern (Figure 8).The diameter of the rings is only about 8 m, which is the smallest of any artificial coffee-ring structures in the world.Furthermore, these new patterns are distributed uniformly over the whole macroscale substrate, which is meaningful for real applications.As a result of this finding, the realization of very small biosensors is possible, which means it becomes possible to pack thousands, or even millions, of small microbiosensors onto a single lab-on-a-chip device in a one-step process, allowing a large number of medical or chemical analyses to be performed on a single chip.

Electrochemical Method.
Electrochemical method is a way to utilize chemical reactions which take place in a solution at the interface of an electron conductor.These reactions involve electron transfer between the electrode and the electrolyte or species in solution.He et al. fabricated ZnO thin films with diverse nanostructure, including nanodot, nanowire-and nanoflowers on zinc foils by a simple and rapid electrochemical anodization method [69].Under the dc or ac electric field, the electroinduced surface wettability conversion from the superhydrophobic to hydrophilic state was observed and the generation of surface defective sites on ZnO films under electric field was used to explain the transition mechanism.Electroinduced surface wettability conversion is also studied by other researchers [70,71].It  related to the density of these one-dimensional structures.Briefly, a surface with low density will render the surface with relatively high contact angle.

Conclusions
In this review, we have tried to represent most of the advances in the domain of ZnO superhydrophobic surfaces.We focused on the theory advances, synthesis methods, and the multifunctional characteristic of different ZnO structures.The synthesis approaches reviewed were separated into several types, most of which could realize various surface morphologies with controllable structures.And the related applications benefit from these structures has also been included.This review allows readers to have an easy access to recent advances in this domain in order to find possible approaches to produce such surfaces or to find new functions for interesting applications.

Figure 1 :Figure 2 :
Figure 1: (a) A force balance along the horizontal direction at three-phase contact line is considered.(b) A contact angle at which thermodynamic energy becomes minimum can be derived when a liquid moves by .

Figure 3 :
Figure 3: Liquid drop on the slanted solid plate.A liquid drop begins to slide down when the slant angle of the plate is larger than the threshold angle at which gravity balances with contact angle hysteresis.

Figure 4 :
Figure 4: FESEM micrographs of ZnO nanoclustered film (a) with 433 nm thickness, and (b) corresponding AFM 2D roughness profile.(c) The FESEM image of 1300 nm optimized thickness with its (d) 3D morphology.The thickness of ZnO nanostructured coating was controlled by sputtering time.Copyright 2012, Elsevier.

Figure 7 :Figure 8 :
Figure 7: SEM images of the prepared ZnO films on zinc substrate: ((a)-(c)) overview of the ZnO film surface at various magnifications; (d) overview of the cross section.Copyright 2011, Elsevier.