Bio-Inspired Titanium Dioxide Materials with Special Wettability and

Review pubs.acs.org/CR

Bio-Inspired Titanium Dioxide Materials with Special Wettability and Their Applications Kesong Liu,†,∥ Moyuan Cao,† Akira Fujishima,§ and Lei Jiang*,†,‡ †

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China § Research Institute for Science and Technology, Photocatalysis International Research Center, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ∥ Institute for Superconducting and Electronic Materials, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia 7. Titanium Dioxide Surfaces with Switchable Wettability 7.1. UV Light 7.2. Visible Light 7.3. Thermal Stimulus 7.4. Electric Potential 7.5. Multi-Stimuli 7.6. Switchable Oil Wettability 8. Application of Titanium Dioxide Materials with Special Wettability 8.1. Antibacteria 8.2. Anticorrosion 8.3. Antifogging 8.4. Biomedical 8.5. Device 8.6. Liquid Transportation 8.7. Liquids Separation 8.8. Offset Printing and Liquid Reprography 8.9. Self-Cleaning 8.9.1. Superhydrophilicity/Photocatalysis-Induced Self-Cleaning 8.9.2. Superhydrophobicity-Induced SelfCleaning 8.10. Site-Selective Functional Patterning 8.11. Water Condensation 8.12. Agricultural and Environmental Fields 9. Conclusion Author Information Corresponding Author Notes Biographies Acknowledgments ABBREVIATIONS References

CONTENTS 1. Introduction 2. Fundamental Theories 2.1. UV Light-Induced Superhydrophilicity of TiO2 2.2. Typical Wetting Theories 3. Superhydrophilic Titanium Dioxide Materials 3.1. UV Light-Induced Superhydrophilicity 3.2. Visible Light-Induced Superhydrophilicity 3.3. Electrowetting-Induced Superhydrophilicity 3.4. Structure-Induced Superhydrophilicity 3.5. Thermo-Induced Superhydrophilicity 4. Superhydrophobic Titanium Dioxide Materials 4.1. Low-Adhesive Superhydrophobicity 4.2. High-Adhesive Superhydrophobicity 4.3. Superhydrophobic Surfaces with Switchable Water Adhesion 4.3.1. Light-Induced Switchable Adhesion 4.3.2. Surface Chemical Compositions-Induced Controllable Adhesion 4.3.3. Surface Structures-Induced Tunable Adhesion 5. Superhydrophilic−Superhydrophobic Patterns 5.1. Photomask-Induced Wetting Patterns 5.2. Inkjet-Induced Wetting Patterns 5.3. Rewritable Wetting Patterns 6. Superoleophobic Titanium Dioxide Materials 6.1. Superoleophobicity in Air 6.2. Underwater Superoleophobicity © 2014 American Chemical Society

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Special Issue: 2014 Titanium Dioxide Nanomaterials Received: December 17, 2013 Published: June 23, 2014 10044

dx.doi.org/10.1021/cr4006796 | Chem. Rev. 2014, 114, 10044−10094

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1. INTRODUCTION Titanium dioxide (TiO2) is one of the most widely used nanomaterials in our daily life. In 1972, Fujishima and Honda reported the photo electrolysis of water into H2 and O2 utilizing an electrochemical cell in which the TiO2 single-crystal electrode is connected with a Pt electrode.1 This is analogus with the natural photosynthesis that produces oxygen through oxidizing water and reducing carbon dioxide using sunlight, where solar energy is converted into chemical energy.2 Since that time, photocatalysis has received considerable attention owing to its important applications in the conversion of light energy into useful chemical energy.3 In 1997, Fujishima et al. first reported the photogeneration of a superamphiphilic (both superhydrophilic and superoleophilic, where the contact angle of water and oil on a surface is almost 0°, respectively) TiO2 surface under UV light irradiation, showing self-cleaning and antifogging characteristics.4 This breakthrough work expanded the research field of TiO2 materials and marked the beginning of a new era in TiO2-based self-cleaning materials. Since then, an important effort has been focused on the understanding of the fundamental mechanism of this novel function and on the development of selfcleaning materials for a wide range of applications in energy, environmental, and industrial fields, resulting in the generation of new markets. Although photocatalysis and photoinduced superhydrophilicity can take place simultaneously on the same TiO2 surface, they are intrinsically different processes.5 In recent years, environmental pollution and damage on a global scale have emerged as a serious issue. The viable environmental cleanup has attracted a great deal of attention to achieve important breakthroughs in the design of advanced materials and in the development of new technology. Now, a variety of TiO2-based materials have been commercialized arising from their unique photoinduced properties. Furthermore, these commercial products demonstrate their importance in the environmental cleanup.5−26 Nature is a school for scientists and engineers. After billions of years of evolution, creatures in nature exhibit almost perfect structures and functions. Nature has long served as a source of inspiration for human beings to design advanced materials and to develop new technology.27−29 Surface wettability is an important property for plants and other biomaterials, which has an effect on photosynthesis, water absorption, pathogen infection, and other physiological process. In nature, many biological materials exhibit fascinating superwettability, such as lotus leaves, rice leaves, peanut leaves, red rose petals, cicada wings, butterfly wings, mosquito compound eyes, fly eyes, gecko feet, Nepenthes pitcher plants, Salvinia molesta floating leaves, desert beetles, spider silk, cactus, fish scales, and others.30−46 The special surface wettability can be ascribed to the cooperation of surface multiscale structures and surface chemistry. Wettability of a solid surface by a liquid is a very important aspect of materials science and surface chemistry, which is vital for practical applications in the fields of industry, agriculture, and daily life. Inspired by superwetting biomaterials, a great number of innovative synthesis strategies have been proposed to fabricate novel and advanced materials with superwettability through the integration of surface geometrical structures and surface chemical compositions.47−77 In the past few years, a great deal of work has been devoted to constructing novel materials through the integration of photoinduced characteristics of TiO2 and special surface wettability of biomaterials. This research direction has attracted wide scientific

attention for both fundamental research and practical applications in the fields of energy, environment, medicine, industry, and others. This review will focus on the recent developments in the mechanism, fabrication, and application of bioinspired TiO2 materials with special wettability, which is composed of the following nine sections. A brief summary of fundamental theories including the mechanism of light-induced superhydrophilicity of TiO2 and typical wetting theories is presented in section 2. Section 3 describes the recent progress in the fabrication of superhydrophilic TiO2 materials driven by UV light, visible light, electric potential, structure, and thermo. Lowadhesive superhydrophobic TiO2 materials, high-adhesive superhydrophobic TiO2 materials as well as the superhydrophobic TiO2 surfaces with switchable water adhesion are shown in section 4. A brief summary of the superhydrophobic-superhydrophilic TiO2 pattern is presented in section 5. Section 6 summarizes the superamphiphobic (both superhydrophobic and superoleophobic, where the contact angle of water and oil on a solid surface is larger than 150°, respectively) TiO2 materials in air and underwater superoleophobic TiO2 surfaces. A comprehensive overview on TiO2 surfaces with switchable wettability driven by UV light, visible light, thermal stimulus, electric potential, and multistimuli is shown in section 7. The switchable oil wettability of TiO2 materials is also discussed in this section. In section 8, practical applications of bioinspired TiO2 materials with special wettability in antibacteria, anticorrosion, antifogging, biomedical, device, liquid transportation, liquids separation, offset printing, liquid reprography, self-cleaning, site-selective functional patterning, water condensation, and agricultural and environmental fields are described. In the final section, a brief summary and future outlook on the development of bioinspired TiO2 materials with special wettability are addressed.

2. FUNDAMENTAL THEORIES 2.1. UV Light-Induced Superhydrophilicity of TiO2

In 1997, Fujishima et al. first reported the UV light-induced superamphiphilicity of TiO2 surfaces for both water and oil liquids.4 Before UV irradiation, the TiO2 film exhibited a water contact angle of about 72°. After UV irradiation, water droplets spread completely on the film, resulting in a contact angle of about 0° (Figure 1). A similar behavior was also found for oil liquids, such as glycerol trioreate and hexadecane. The wettability of TiO2 surfaces was reversible between superhydrophilicity and hydrophobicity (here, the description of hydrophobicity is relative to that of superhydrophilicity under UV irradiation; the conventional definition of hydrophobicity is the water contact angle larger than 90°) under the alternation of UV irradiation and long-term storage in the dark. The switchable wettability can be observed on both anatase and rutile TiO2 surfaces in the form of either single crystals or polycrystals, independent of their photocatalytic activities. After the discovery of light-induced superamphiphilicity of TiO2 surfaces, a great deal of work has been devoted to explicating the formation mechanisms of this unique photoinduced wetting behavior.5,22 In this section, we mainly discuss the photoinduced hydrophilicity mechanism of pure-TiO2. The mechanism for TiO2-based composites with the photoinduced wettability conversion will be presented in the following section 3. According to the friction force microscopy study of a rutile TiO2 (110) single crystal, it was observed that, before UV illumination, TiO2 surfaces exhibited microscopically homogeneous wettability.4 However, after UV illumination, a pattern 10045


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amphiphilic characteristic of TiO2 surfaces under UV irradiation. The FTIR study exhibited the reversible growth and decay of a peak (3695 cm−1) assigned to the formation of hydroxyl groups chemisorbed on a surface defect site.78 The decrease of all the bands was observed after the storage of TiO2 in dark, correlated with both the hydroxyl desorption at the defect sites and the molecular water desorption. After UV light irradiation, the 1623 cm−1 band decreased but the 3695 cm−1 band increased, showing the reduction of the amount of adsorbed molecular water and the enhancement in the amount of adsorbed dissociated water. The study of ultrasonic treatment on amphiphilic TiO2 surfaces in pure water further demonstrated the above mechanism.79 It was found that the hydrophilic−hydrophobic conversion is not due to the contamination of the surface but due to the change of the surface physical and chemical structure. Furthermore, ultrasonication greatly facilitated the reconversion process from amphiphilicity to hydrophobicity, arising from the generation of OH radicals which possess strong oxidizing power and thus accelerate the reconversion process. Subsequent UV irradiation resulted in the surface hydrophilicity again. A thorough study of surface wettability conversion on single-crystal TiO2 surfaces revealed that the UV illumination-induced superhydrophilicity can be attributed to the photoreduction of Ti4+ to Ti3+ at definite sites on TiO2 surfaces, resulting in the preferential adsorption of hydroxyl groups on corresponding oxygen vacant sites.80 It has been demonstrated that oxygen vacancies played an important role in the water adsorption process on TiO2 single-crystal surfaces.81,82 The long-term storage in dark-induced wetting reconversion from superamphiphilicity to hydrophobicity was ascribed to the replacement of the adsorbed hydroxyl groups by oxygen in air, which returned the surface geometric and electronic structures similar to the native TiO2 surface. Furthermore, various TiO2 single-crystal surfaces exhibited different wetting behaviors arising from their distinct surface structures, demonstrating the importance of oxygen bridging sites in the surface wettability conversions.80,83 The (100) and (110) faces of TiO2 single crystals presented the similar UVinduced wetting conversion rate from hydrophobicity to hydrophilicity, owing to the presence of Ti3+ sites substantiated by the XPS study.80,83 However, the (001) face exhibited a completely different wetting behavior from both the (110) and (100) faces, which need a long time to achieve UV-induced hydrophilicity owing to the absence of reactive bridging site oxygen on the face. The effect of repeated UV irradiation on the photoinduced wettability conversion of rutile single crystals has been further investigated.84 It was found that the hydrophilicizing rate was increased by the repeated UV light irradiation. This effect was more remarkable on (001) surfaces than on (110) surfaces, which can be ascribed to the differences of oxygen vacancy generation and the degree of resultant structural distortion by the oxygen replacement with the dissociated water. This implied that the photoinduced superhydrophilicity of TiO2 films was a kind of photocorrosion process on the surface. In addition to the single crystal TiO2, the polycrystalline TiO2 thin film also exhibited photoinduced amphiphilicity.85 The initial contact angles for water and hexadecane on TiO2 polycrystalline surface were about 40° and 10°, respectively. Under UV irradiation with 1 mW/cm2 for 2 h, a superamphiphilic surface was achieved for both water and hexadecane with a contact angle of about 0°, which can be maintained for 10 h. However, it was found that, further UV irradiation resulted in a gradual increase of the hexadecane contact angle to 25°, showing a hydrophilic-oleophobic behavior. The photoinduced amphi-

Figure 1. Switchable wettability of TiO2 surfaces. (a) A hydrophobic surface before UV irradiation. (b) A superhydrophilic surface after UV irradiation. (c) Exposure of a hydrophobic TiO2-coated glass to water vapor. The formation of fog (small water droplets) hindered the view of the text on paper placed behind the glass. (d) Creation by UV irradiation of an antifogging surface. The high hydrophilicity prevents the formation of water droplets, making the text clearly visible.4 Reprinted with permission from ref 4. Copyright 1997 Nature Publishing Group.

with hydrophilic (bright) and oleophilic (dark) areas was spontaneously formed on TiO2 surfaces (Figure 2). Hydrophilic domains possessed a regular rectangular shape with 30−80 nm in size, aligned along the [001] direction of the (110) single crystal surface. The formation of the nanoscale pattern with hydrophilic and oleophilic phases resulted in the macroscopically super-

Figure 2. (a) FFM image (5 × 5 μm2) for a rutile TiO2 (110) surface before UV illumination; (b) FFM image (5 × 5 μm2) of the same surface after UV illumination; (c) a medium scale FFM image (1000 × 1000 nm2) of the framed area in panel b; (d) a higher-magnification topographic image (240 × 240 nm2) of the framed area in panel c. For panel d, the sample stage was rotated 45° from the position in panel c.78 Reprinted with permission from ref 78. Copyright 1998 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 10046


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philic conversion can be attributed to the variations of hydrophilicizing rate of TiO2 grains, while the photoinduced hydrophilic-oleophobic property should result from a redistribution of the nanoscale hydrophobic (oleophilic) domains after further UV illumination, which played an important role for the self-cleaning performance of the TiO2 thin films. Both amphiphilic and hydrophilic-oleophobic properties were not observed on other oxide surfaces, which are unique for the TiO2 polycrystalline thin film. In order to further elucidate the photoinduced hydrophilic conversion mechanism, polycrystalline TiO2 was used as the working electrode in an aqueous electrolyte under potential-controlled conditions.86 It was proposed that the photoinduced hydrophilicity is initiated by two-hole trapping by surface lattice oxygen, which resulted in the formation of an oxygen defect. Water molecules spontaneously adsorbed at the defect sties dissociatively. This process is similar to the photocorrosion approach of metal oxides in aqueous solution. The proposed photoinduced hydrophilic mechanism is on the basis of the assumption that topmost surface atoms have been removed both in the neutral aqueous solution and in air. Although a variety of TiO2 thin films with the photoinduced hydrophilic conversion have been fabricated, no quantitative methods have been developed to evaluate this conversion ability. Utilizing the reciprocal of the contact angle, a quantitative evaluation method was proposed to investigate the relations of the polycrystalline TiO2 hydrophilic conversion with various parameters such as the UV wavelength, intensity, and hole scavenger.87 It was suggested that UV irradiation resulted in the reconstruction of surface hydroxyl groups (Figure 3a−c). The density of surface hydroxyl groups is correlated with the reciprocal of the contact angle. Photoexcited electrons are captured by the molecular oxygen and photoexcited holes diffuse to TiO2 surfaces, being trapped at lattice oxygen atoms. Then, the binding energy between Ti and the lattice oxygen was weakened. The bond was interrupted by the water molecule, resulting in new hydroxyl groups. During the dark storage, hydroxyl groups gradually desorbed from the surface in the form of H2O2 or H2O + O2. This implied that the photocatalytic oxidation process and the photoinduced wetting conversion proceed competitively on the TiO2 surface under UV illumination. For the photocatalysis process of bare TiO2 (Figure 3d), the adsorption of a photon with energy equal to or larger than its band gap (about 3.2 eV) resulted in the generation of charge carriers (holes and electrons). Although most of these charge carriers can rapidly self-recombine, some of them migrate to the TiO2 surface and initiate redox reactions with adsorbate, where the trapped hole htr+ and electron etr− react with donor and acceptor molecules, respectively. The trapped holes can react with water or OH− on the photocatalyst surface to generate hydroxyl radicals (•OH). The photogenerated holes and the hydroxyl radicals can subsequently oxidize organic species adsorbed on TiO2 surfaces. The trapped electrons combine with molecular oxygen in air to produce superoxide radical anion (O2•−) and other further reactive oxygen species, which contribute to the photocatalytic degradation of pollutants on TiO2 surfaces. For the optimal state, the photogenerated hole and electron pairs should be separated as far as possible to enhance the photocatalytic performance. Otherwise, hole and electron pairs can also be trapped at bulk trapping sites and recombine with an undesired release of heat. It was found that the electrochemical potential has an effect on the surface wettability even in the dark, especially for amorphous TiO2 films.88 The amorphous TiO2 electrodes surface became

Figure 3. (a−c) Surface reconstruction on TiO2 during the reversible hydrophilic changes. (a; before UV irradiation) The OH group is bound to oxygen vacancy, (b; at the transition state) the photogenerated hole is trapped at the lattice oxygen, and (c; after UV irradiation) new OH groups are formed.87 Reprinted with permission from ref 87. Copyright 2003 American Chemical Society. (d) Processes occurring on the bare TiO2 particle after UV excitation.5 Reprinted with permission from ref 5. Copyright 2008 Elsevier.

highly hydrophilic under cathodic polarization without UV light irradiation. The contact angle of amorphous TiO2 electrodes decreased in the dark by applying negative voltage more than −0.5 V. The hydrophilic conversion rate increased with a larger negative potential. At −1.0 V, the contact angle reached nearly 0° after 30 min. It was proposed that the increase in hydroxyl groups arising from electron addition to the near-band-edge trap sites accompanied by proton intercalation resulted in the hydrophilic conversion of amorphous TiO2 under cathodic conditions, which is different from the photoinduced hydrophilic conversion mechanism on TiO2 surfaces. It is well-known that TiO2 possesses two inherent photoinduced properties, photocatalysis and photoinduced superhydrophilicity. Although both phenomena can take place simultaneously on the same TiO2 surface, they exhibited intrinsically different processes and mechanisms. Several research groups initially proposed that the photoinduced hydrophilicity results from the photocatalytic removal of hydrocarbon contamination films from TiO2, which is an inadequate mechanism.89,90 Similar to TiO2, SrTiO3 is an efficient photocatalyst for the production of gaseous hydrogen from water and for the decomposition of various organic compounds. Furthermore, the electronic structure of SrTiO3 resembles that of TiO2. Through the comparison of TiO2 and SrTiO3 polycrystalline films, it was found, although both types of films possess almost the same photocatalytic oxidation activity, the SrTiO3 film did not exhibit photoinduced hydrophilicity 10047


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under UV irradiation.91 This work demonstrated the inherent photoinduced hydrophilicity of TiO2 is not induced by the photocatalytic oxidation of organic compounds adsorbed on the surface. A variety of other metal oxide thin films have also been investigated for their photoinduced hydrophilicity under UV illumination and photocatalytic ability to decompose adsorbed dye.92,93 On the basis of the resultant photoinduced behavior, these metal oxides can be classified into the following four categories: (1) both photocatalysis and photoinduced hydrophilicity are observed for TiO2, SnO2, and ZnO; (2) only the photocatalytic property is observed for SrTiO3; (3) only photoinduced hydrophilicity is observed for NaNbO3, WO3, and V2O5; (4) and neither photoinduced property is observed for CeO2, CuO, MoO3, Fe2O3, Cr2O3, and In2O3. This work further demonstrated that photoinduced hydrophilicity is not caused by the photocatalytic oxidation of adsorbed organic compounds, but is attributed to structural changes at metal oxide surfaces. Furthermore, for the metal oxides (TiO2, SnO2, ZnO, WO3, and V2O5) possessing a photoinduced hydrophilicity effect, a UVdriven reversible switching between superhydrophilicity and superhydrophobicity was achieved through the construction of optimal surface structures.94−98 These results are one of the most convincing evidence with regard to distinguishing photocatalysis and photoinduced hydrophilicity.

in a deviation of the contact angle from the value established by Young’s equation. Therefore, two wetting models are successively proposed to describe the relationship between surface roughness and the apparent contact angle of a solid surface, Wenzel and Cassie−Baxter models. In 1936, Wenzel first appreciated that roughness resulted in an amplification of the wetting properties.102 A model was proposed by Wenzel to clarify the relationship between the contact angle and surface roughness using the following equation:

cos θ W = r cos θY

where θW is the apparent contact angle in the Wenzel state, θY is the Young’s contact angle, and r is the surface roughness factor, the ratio between the actual surface area and the apparent surface area of a rough surface. The basic assumption in the Wenzel model is that liquids completely fill the grooves of rough surfaces (Figure 4b). The Wenzel equation predicts that surface roughness enhances both hydrophilicity and hydrophobicity, which is dependent on the nature of the corresponding surface. If the surface roughness factor r is larger than 1, a hydrophilic surface (θY < 90°) becomes more hydrophilicity (θW < θY) with an increase in surface roughness. Conversely, a hydrophobic surface (θY > 90°) will present increased hydrophobicity (θW > θY). Although these tendencies can be generally (but not always) observed, in some cases, the Wenzel model is not sufficient when dealing with a heterogeneous surface.48,103−106 In 1944, Cassie and Baxter first investigated the effect of chemical heterogeneities on the equilibrium contact angle.107 A model was proposed to clarify the relationship between chemical heterogeneities and the contact angle, which can be expressed by the following equation:

2.2. Typical Wetting Theories

The wetting behavior of solid surfaces by a liquid is usually expressed by the contact angle θ. The contact angle concept is of fundamental importance in all solid−liquid-fluid interfacial phenomena. For a liquid droplet on a solid substrate, the wettability is determined by the surface free energy of solid surfaces. In 1805, Young first described the intrinsic contact angle of a liquid droplet on an ideal (i.e., rigid, flat, chemically homogeneous, insoluble, and nonreactive) solid surface.99 Young’s equation is one of the oldest and most used equations of physics and chemistry (Figure 4a). For an ideally flat and

cos θCB = f1 cos θ1 + f2 cos θ2

(3)

where θCB is the apparent contact angle in the Cassie−Baxter state, θ1 and θ2 are contact angles of phase 1 and phase 2, respectively, and f i is the fractional area of the surface with a contact angle of θi (f1 + f 2 = 1). Liquids are assumed to only contact the top of solid asperities, and air pockets are presumed to be trapped underneath the liquid, demonstrating a composite surface (Figure 4c). If only air was located between the solid and the liquid, the cosine of the contact angle is −1. In this case, the Cassie−Baxter equation can be derived as

Figure 4. Typical wetting models of a droplet on solid substrates. (a) A liquid drop on a flat substrate (Young’s model). (b) Wetted contact between the liquid and the rough substrate (Wenzel’s model). (c) Nonwetted contact between the liquid and the rough substrate (Cassie’s model).56 Reprinted with permission from ref 56. Copyright 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

cos θCB = fs (cos θ + 1) − 1

(4)

where θCB is the apparent contact angle in the Cassie−Baxter model and fs is the fractional area of the given solid surface with a contact angle of θ. Both Wenzel and Cassie−Baxter models are good approximations of contact angles on imperfect surfaces, but both equations give a more qualitative than quantitative evaluation on the relationship between contact angles on rough and flat surfaces.108 It is still a challenge for the above wetting models to explain the exact mechanism between droplets and solid surfaces in the practical conditions. In the past decades, a series of refined theories and models have been proposed to elucidate the wetting principles, especially for the superhydrophobic surfaces.108−120 These models and theories provide an important avenue to design and predict functional surfaces with special wettability. Contact angle hysteresis is an important characteristic for a solid−liquid interface, which is ubiquitous in nature and plays a crucial role in various industrial processes.121−123 The difference between advancing (θadv) and receding (θrec) contact angles is

homogeneous solid, the classical Young’s equation can be written as

γsg = γsl + γlg cos θY

(2)

(1)

where θY is the Young’s contact angle and γsg, γsl, and γlg refer to the interfacial surface tensions with g, l, and s as gas, liquid, and solid, respectively. It should be noted that Young’s equation describes a system in its state of thermodynamic equilibrium.60,100,101 For the ideal solid surface, chemical heterogeneity, surface roughness, surface reconstruction, swelling, and dissolution are not considered. In fact, solid surfaces are far from this ideal situation. For example, surface roughness has an important effect on the wettability of a solid surface, which results 10048


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defined as contact angle hysteresis (Δθ = θadv − θrec). When the drop is inflated, the contact line first remains stuck before it suddenly jumps above a critical volume. The maximum observed angle is the advancing contact angle θadv. Conversely, when the drop is deflated, the contact line will not change until the droplet begins to recede. The minimum angle is the receding contact angle θrec. In 1962, Furmidge first proposed a simple equation supported by experimental results to explain the retention of droplets on inclined planes, which can be expressed as124 mg (sin α)/w = γAL(cos θrec − cos θadv)

same area fractions, they will have different three-phase contact line structures (Figure 5).146 A continuous three-phase contact

(5)

where m is the mass of the drop, g is the force due to gravity, α is the angle of tilt of the surface necessary to produce sliding of the drop, γAL is the surface tension of the liquid drop, w is the width of the drop, θadv and θrec are the advancing and receding contact angles, respectively. From this equation, it is clear that a lower droplet weight and smaller differences between advancing and receding contact angles will generate a smaller sliding angle. Usually, in the classical Wenzel regime, liquid will penetrate into the texture and completely wet the asperities, exhibiting a high contact angle hysteresis, in contrast to the well-known lotus effect. In the Cassie regime, the drop will sit on the peaks of the rough surface owing to the presence of the air cushion trapped below the drop, showing a low contact angle hysteresis.51 Surface roughness has a considerable influence on both the contact angle itself and its hysteresis, which has been demonstrated by the experimental and theoretical results.48,125−130 In 1953, Bartell and Shepard appreciated the effect of surface roughness on contact angle hysteresis.126 Further systematic studies were carried out by Johnson and Dettre in 1964 to explain the relationship between contact angle hysteresis and roughness.131,132 They found that, as the roughness increases, the magnitude Δθ of the hysteresis increases at first and goes through a maximum before decreasing suddenly. Usually, the energy gained for surfaces during contact is greater than the work of adhesion for separating surfaces, arising from the so-called adhesion hysteresis.133 Contact angle hysteresis is also related to the adhesion hysteresis.134,135 The force of wet adhesion called the droplet retention force can be expressed as136 F = kγR(cos θrec − cos θadv)

Figure 5. 2-Dimensional (X−Y) representations of two surfaces with the same fraction of water−solid interface area ( f1) and water−air interface area (f 2), but very different contact line structures. (a) A nearly continuous contact line can form (pinning the drop). (b) The contact line is discontinuous and unstable. The dark lines are meant to represent possible contact lines.146 Reprinted with permission from ref 146. Copyright 2000 American Chemical Society.

line is preferable for a surface with a high adhesive property toward water, where the water droplet was pinned. Conversely, a discontinuous and unstable three-phase contact line results in less contact with solid and more with air, showing a low sliding angle. Therefore, the rational design of the solid−liquid−air three-phase contact line is a critical factor to lower the energy barriers for the droplet motion and to promote the sliding behavior of liquid droplets. On the basis of Wenzel and Cassie models, a great deal of work has been devoted to the theoretical mechanism of special wetting surfaces.109 At certain degree of roughness, the Wenzel state and Cassie state could coexist on the same surface depending on how the respective drop was deposited.58,60,155,156 This raises questions of the stability of the two states and the transitions between Wenzel and Cassie states, which have received considerable attention. It has been demonstrated that an irreversible transition can be induced from the low adhesive Cassie state to the high adhesive Wenzel state by compressing a water drop between two identical superhydrophobic plates (Figure 6a).111 This research indicated that the Cassie regime of air-trapping is metastable, while the Wenzel state is stable. Similar wetting transitions from the Cassie to Wenzel state can be triggered by the imposed pressure, evaporation, condensation, vibration, electrical voltage, and hydraulic pressure.156−167 Usually, this transition is accompanied by a decrease in the static contact angle and/or an increase in the contact angle hysteresis. In the past decades, because the Wenzel regime is at the free-energy global minimum state,155 it has been generally regarded that the Cassie-to-Wenzel transition is an irreversible process. However, some recent works demonstrated the transition from the sticky Wenzel state to the nonsticking Cassie state after overcoming the surface energy barriers by application of heat, mechanical vibration, hydrostatic pressure, electrical voltage or current, magnetic field.168−175 For example, reversible switching between the superhydrophobic Cassie state and the hydrophilic Wenzel state can be achieved by the application of electrical voltage and current (Figure 6b).172 In some cases, the wetting behavior of water droplets can be converted from the Wenzel to Cassie state even the former is the energetically more favorable state for the surfaces employed.168 The Wenzel and Cassie models mainly focused on the wettability of a single-scale surface structure on the substrate. However, in nature, superhydrophobic biological surfaces

(6)

where k is a coefficient depending on the shape of the contact line, γ is the surface tension of the liquid drop, R is the radius of the drop, and θadv and θrec are the advancing and receding contact angles, respectively. It was proposed that the contact angle hysteresis is equal to the term corresponding to the adhesion hysteresis plus the term corresponding to the effect of surface roughness.137 Noteworthy, the contact angle hysteresis arises not only from adhesion and the surface geometry (such as surface roughness or heterogeneities), but from molecular interactions between solid and liquid.138 Therefore, in some cases, although the hysteresis value is so low that it becomes basically nonmeasurable, the hysteresis cannot be eliminated completely.139,140 Generally, the surface wetting behavior, especially the adhesion of liquid droplets to solid substrates, is related to the solid−liquid−air three-phase contact line.123,141 This has been appreciated independently by Joanny and de Gennes and by Pomeau and Vannimenus in 1984.142,143 Further theoretical and experimental studies indicated that the shape, length, density, and continuity of the three-phase contact line could affect the mobility of water droplets on surfaces.144−154 For example, although surfaces with different topographies could have the 10049


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Figure 6. (a) Compression of a millimetric water drop between two identical microtextured superhydrophobic surfaces with a small hysteresis. The contact angle first has a plateau value, which corresponds to the air-pocket Cassie regime. With the increase of the pressure, the contact angle then decreases arising from a progressive sinking of the drop inside the texture, showing the Wenzel regime. The imposed pressure resulted in the irreversible transitions from Cassie to Wenzel states.111 Reprinted with permission from ref 111. Copyright 2003 Nature Publishing Group. (b) Demonstration of electrically induced reversible transitions between different wetting states of a liquid on a nanostructured substrate. To induce a transition between a rolling ball and an immobile droplet, a voltage was applied between the droplet (contacted through a Pt wire) and the substrate. To reverse the transition and convert the immobile droplet back to the rolling ball state, a short pulse of electrical current was transmitted through the highly conductive 7.4 μm surface layer of the substrate.172 Reprinted with permission from ref 172. Copyright 2007 American Chemical Society.

possess inherent multiscale structures ranging from nano, micro, to macro-scale.176 Now, a great number of theoretical and experimental studies have demonstrated that multiscale roughness is beneficial for the stabilization of the composite interface and the robustness of superhydrophobicity.177−181 Some recent pioneering works have shown that it is possible to induce superhydrophobic and superoleophobic surfaces on intrinsically hydrophilic and oleophilic materials through the introduction of hierarchical textures with overhang structures.182−185 Overhang structures are able to prevent liquid from penetrating the porous texture arising from the capillary force (Figure 7a). Therefore, the liquid is in contact with a composite surface of air and solid, showing apparent superhydrophobicity and superoleophobicity. In addition, theoretical analysis has demonstrated that multiscale roughness originating from the re-entrant texture can enhance the stability of a composite interface (Figure 7b).186 Following this principle, durable superhydrophobic even superoleophobic materials have been fabricated by utilizing hierarchically structured surfaces with re-entrant texture.187−189 Liquid−solid interfaces and associated wetting phenomena are important topics of surface science and have been widely investigated in the past decades both theoretically and experimentally. Some promising breakthroughs in the advance of wetting theoretical models have been developed on the basis of the classical Wenzel and Cassie models. However, the investigations in this field still have some bottlenecks owing to the complexity and diversity of material surfaces in practical situations. Therefore, further fundamental investigations of the wetting mechanism and the establishment of generalized wetting theoretical models are essential for a rational design and the reproducible construction of multifunctional surfaces with desired wettability.

Figure 7. Schematic cross-sectional profile of liquid in contact with a surface consisting of (a) overhang structures183 and (b) re-entrant structures.186 Reprinted with permission from ref 183. Copyright 2008 American Chemical Society. Reprinted with permission from ref 186. Copyright 2007 American Chemical Society.

3. SUPERHYDROPHILIC TITANIUM DIOXIDE MATERIALS Superhydrophilic materials are ubiquitous in nature and in our daily life. Water droplets on superhydrophilic surfaces can spread or are absorbed very quickly, showing a low water contact angle. As discussed in the above section, superhydrophilic TiO2 surfaces can be fabricated easily through UV irradiation, even displaying superamphiphilic for both water and oil drop10050


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lets.4,78,190 Utilizing this strategy, a wide variety of superhydrophilic TiO2 materials have been prepared, exhibiting environmental friendly applications in self-cleaning, antifogging, and other fields. However, when UV illumination is switched off, TiO2 films tend to lose its superhydrophilicity. In the dark, TiO2 films will return to a more hydrophobic state within minutes to hours, which limits their practical applications. In order to fabricate superhydrophilic TiO2 films without UV irradiation, many different synthesis strategies have been developed in recent years. In this section, we will focus on the recent developments in superhydrophilic TiO2 materials with and without the need of light activation.

The superhydrophilic conversion of TiO2 under UV light irradiation limited to its practical appications. In 2000, Hashimoto et al. reported a transparent TiO2−WO3 thin film by a conventional sol−gel procedure.193 Under very weak UV illumination (1 μW/cm2) by indoor light such as a fluorescent light bulb, TiO2−WO3 thin films exhibited superhydrophilicity. The homogeneous dispersion of amorphous WO3 particles on TiO2 effectively increased the accumulation of photogenerated holes on the TiO2 surface, resulting in a high hydrophilicizing rate. These superhydrophilic films can potentially be used as coatings on indoor products and will exhibit antifogging and selfcleaning properties. Utilizing uniform PMMA microspheres as a template, Yamashita et al. fabricated macroporous TiO2 thin films on quartz substrates through the combination of sol−gel and bottom-up approaches (Figure 9).194 In addition to the

3.1. UV Light-Induced Superhydrophilicity

Since the discovery of TiO2 with superhydrophilicity under UV light irradiation in 1997, a great number of studies have been focused on this material. Fujishima et al. reported double-layered TiO2−SiO2 nanostructured films with superhydrophilicity under UV illumination (Figure 8).191 Although a slight increase in the

Figure 8. Flowchart for the preparation of double-layered TiO2−SiO2 self-cleaning coatings with antireflective properties.191 Reprinted with permission from ref 191. Copyright 2006 American Chemical Society.

water contact angle was observed after the dark storage for 2 weeks, UV illumination lowered the contact angle to almost 0°. The double-layered film was fabricated through the following procedure, (a) layer-by-layer deposition of multilayered SiO2 nanoparticles with PDDA cations, (b) layer-by-layer deposition of multilayered titanate nanosheets with polycations on PDDA/ SiO2, and (c) calcination at 500 °C to remove the polymer and to convert titanate nanosheets into TiO2. The dense TiO2 top layer with photocatalysis and photoinduced superhydrophilicity exhibited self-cleaning. The porous bottom SiO2 layer with low refractive index was decisive for creating antireflective properties. The combination of UV-induced superhydrophilicity, selfcleaning, and antireflection provided promising outdoor uses. They also investigated the effect of SiO2 addition on UV lightinduced superhydrophilicity of TiO2 thin films. It was found that the SiO2 addition less than 30 mol % has a suppressive effect on the transformation of TiO2 from anatase to rutile and on the anatase crystal growth during the calcination process, which resulted in the enhancement of photocatalytic activity and the capability of holding absorbed water during UV irradiation. Recently, Fujishima et al. investigated the effect of several experimental parameters (including wetting, light intensity, heating, spectral variation of the actinic light, and surface acidity) on UV-induced superhydrophilic conversion of TiO2 surfaces.192 Temperature dependence and the effect of the surface acidity on the TiO2 surface hydrophilicity indicated the importance of the multilayer hydrate structure in both the original hydrophilicity and the direction of the photoinduced wetting conversion.

Figure 9. Schematic diagram of the procedures for preparation of macroporous TiO2 thin films.194 Reprinted with permission from ref 194. Copyright 2011 Royal Society of Chemistry.

enhanced photocatalytic activity and high transparency, these macroporous films showed good photoinduced superhydrophilicity after a short period of UV light irradiation. The enhancement of surface superhydrophilicity was attributable to both the formation of fine roughness on film surfaces and the increase of Ti−OH content by the construction of macropores. Lee et al. reported the fabrication of highly hydrophobic TiO2/ PMMA composite films.195 When the volume ratio of TiO2 to PMMA is between 35 and 50 vol %, the mixture of two hydrophilic materials converted very hydrophobic. This can be ascribed to the preferential orientation and attachment of a carbonyl group of a polymer molecule to TiO2 nanoparticles surfaces. After UV irradiation, the film surface turned out to be superhydrophilic arising from the porous structure and the photocatalytic decomposition of PMMA. Rathouský et al. reported crystalline TiO2 films with a stable superhydrophilic state under 1 mW/cm2 UV light irradiation.196 After storing in dark for 17 h, the resultant films remained highly hydrophilic with a contact angle of 15°, which can be attributed to the presence of 3D interconnected porosity in the films. Sol−gelderived nanocrystalline TiO2 thin films were fabricated by Falaras et al. on glass substrates.197 Under UV irradiation, these films exhibited superhydrophilicity, which is dependent on the crystalline size and surface structures. Utilizing a low-temper10051


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Figure 10. Schematic image showing the physical changes during calcination of (a) PDAC/SiO2 and (b) TiO2/SPS multilayers. In both figures, the round spheres and chains represent the nanoparticles and polyelectrolytes used in each system, respectively. (c) The same three films consisting of five alternating regions of TiO2 and SiO2 nanoparticles on the glass substrates with colors of purple, blue, and yellow from left to right. The width of each glass slide is 25 mm.204 Reprinted with permission from ref 204. Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

after sun-illumination, which can be attributed to the formation of microcracks on composite surfaces resulting from the polymer degradation by TiO2 photocatalysis. In nature, nano-ordered structures on some biomaterial surfaces not only contributed a structural color but also endowed the surface with special wettability such as superhydrophilicity.71 Superhydrophilic films with structural color could be used as selfcleaning pigments. Inspired by the structural color in nature, Gu et al. prepared inverse opal TiO2 films with three-dimensionally ordered structures.203 The color changed with the size of the polystyrene spheres used in the opal template and could be regulated from blue to red. These bioinspired TiO2 films exhibited structural color, photocatalysis, and stable photoinduced superhydrophilicity, demonstrating the multifunctional property. The superhydrophilicity of TiO2 films could remain for over one year in the dark, arising from the special nanoporous structures. Cohen et al. also reported porous TiO2−SiO2 thin films with structural color through the polyelectrolyte-assisted layer-by-layer deposition with subsequent calcination to remove the polymer (Figure 10).204 The large contrast of the refractive indices between nanoporous TiO2 and SiO2 resulted in the structure color effect. In addition to structural color, superhydrophilicity and self-cleaning were also observed, arising from the presence of nanopores and photocatalytic TiO2 nanoparticles inside the TiO2−SiO2 Bragg stacks. The nanoporosity-driven superhydrophilicity was stable at room temperature even in the dark for at least four months.

ature aerosol-deposition technique, Yoon et al. prepared superhydrophilic, high-transmittance TiO2 films on a glass substrate without using a carrier solvent.198 The water contact angle decreased with increasing film thickness. After UV irradiation for 5 h or calcination at 400 °C, these films converted to superhydrophilicity. Rare earths-doping can change the physical and chemical properties of TiO2 films. Yang et al. reported the synthesis of CeOx-SiO2−TiO2 thin films on glass by a sol−gel dip-coating method.199 The addition of SiO2 and Ce greatly improved the hydrophilicity of TiO2 films with a water contact angle less than 10°. After UV irradiation for 30 min, these composite films exhibited superhydrophilicity with a water contact angle of 3°, arising from the enhancement of the surface acidity, defect sites, and the decrease in grain size. Miyauchi et al. reported superhydrophilic and transparent thin films of TiO2 nanotube arrays through the vacuum deposition followed by a hydrothermal treatment in an aqueous NaOH solution.200 After UV irradiation, these films presented long-term superhydrophilicity even after the dark storage for more than three months owing to their special surface nanostructures. Koumoto et al. fabricated amorphous TiO2 thin films on self-assembled monolayers through the combination of the peroxotitanate-complex deposition and liquid-phase deposition.201 After UV irradiation for 1 min, amorphous TiO2 thin films exhibited superhydrophilicity. This indicated that the UV-light-induced superhydrophilicity was not specific to crystalline TiO2. The surface wettability of amorphous TiO2 thin films can be switched between superhydrophilicity and hydrophobicity by alternatively exposing the surface under UV light and drying in an atmosphere filled with organic gases. The wettability conversion can be ascribed to the transformation of Ti−OH to the Ti−O−Ti groups. Zhou et al. prepared TiO2-based nanocomposite coatings by mixing TiO2 nanoparticles with silica and methoxysilane group-bearing styrene-co-acrylate oligomer and subsequent curing with APS.202 The resultant nanocomposite coatings exhibited stable superamphiphilicity with self-cleaning

3.2. Visible Light-Induced Superhydrophilicity

TiO2 is a wide-band gap semiconductor with 3.0−3.2 eV of band gap energy that requires an excitation light wavelength range shorter than ca. 400 nm (such as UV light), which limits its light harvesting capacity of the incoming solar radiation at the surface of the Earth. Doping has been demonstrated as an effective solution to enhance the visible light photointroduce performance of TiO2. Therefore, various doping techniques have been developed to lower the band gap. 10052


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photocatalytic reactivity of quantum-sized TiO2 particles.213,214 It was found that dopant metal ions with closed shells have little effect on reactivity. Furthermore, the type, nature, and amount of transition metal are the important factors that affect the photocatalytic degradation of organic species under visible region. The doping of transition metals or their oxides into TiO2 can also influence the surface wettability and then improve the self-cleaning effect in the visible light region.72 Mokhtarimehr et al. prepared V doped TiO2−SiO2 thin films through the sol−gel dip coating method.215 It was found that the V doping shifted the absorption edge of TiO2 to the visible region. Upon visible light irradiation, V doped TiO2−SiO2 films showed superhydrophilicity due to the V dopant-induced enhancement of charge pair separation efficiency and the increase of the surface hydroxyl group density. Yang et al. fabricated Y 2 O 3 doped TiO 2 nanocomposite films by a sol−gel dip-coating method.216 After daylight lamp irradiation for 60 min, the resultant nanocomposite films exhibited superhydrophilicity, which can be partly attributed to the presence of visible light exciting photoinduced pairs of electrons and holes. The incorporation of Y2O3 resulted in the surface property changes of TiO2 films. For the Y2O3 doped TiO2 nanocomposite, Ti4+ sites are substituted by Y3+ ions. The excess negative charge will rapidly trap the photoinduced electron, therefore, photogenerated holes possess more opportunities to combine with H2O adsorbed on the film surface. The surface hydroxyl should also be considered for the generation of visible light-induced superhydrophilicity, which can combine with water molecules to form the hydrogen bond. Furthermore, the optical properties of nanocomposite films in the visible light region have been obviously improved owing to a red shift of the absorption edge. These Y2O3 doped TiO2 films exhibited promising applications in self-cleaning, antifogging, and other potential fields.

Nitrogen (N) can be easily introduced in TiO2 structures to achieve visible light-induced photocatalysis and superhydrophilicity, arising from its comparable atomic size with oxygen, high stability, and small ionization energy.205 Hashimoto et al. first evaluated the visible light induced hydrophilicity on Nsubstituted TiO2 films (Figure 11).206 It was found that the

Figure 11. Changes in water contact angles of N-doped TiO2 and TiO2 thin films under visible light irradiation.206 Reprinted with permission from ref 206. Copyright 2003 Royal Society of Chemistry.

hydrophilicity was enhanced by increasing the degree of nitrogen substitution at oxygen sites under visible light irradiation with an intensity of 0.2 mW/cm2. This can be attributed to the increase in the absorbed photon number rather than the changes in the band structure. Parkin et al. also fabricated N-doped TiO2 visible light photocatalysts by a sol−gel method, which can be further enhanced using Ag nanoparticle islands.207 Silver nanoparticles were deposited on TiO2 surfaces through UV photoassisted reduction of silver nitrate solution. Visible light induced superhydrophilicity was observed in the N-TiO2 samples as well as the Ag−N−TiO2 but not in the pure TiO2 samples. The presence of nitrogen in the film has a very positive effect on the decrease of the water contact angle. Shang et al. reported N and F codoped TiO2 nanotube arrays with dispersed palladium oxide nanoparticles.208 These codoped nanotube arrays exhibited increased visible light absorption, fast superhydrophilicity conversion, and enhanced photocatalytic performance. Without visible light illumination, the water contact angle of TiO2 nanotube arrays is about 13°. After visible light (>400 nm) irradiation for less than 4 min, a superhydrophilic conversion was achieved. Such a fast visible light induced superhydrophilicity is an indication of the superior photocatalytic property of codoped TiO2 nanotube arrays, which has promising applications in selfcleaning. Kontos et al. also fabricated N−F codoped TiO2 films through dip coating of a modified TiO2 sol−gel consisting of a nitrogen precursor and a nonionic fluorosurfactant.209 The N−F codoped TiO2 films showed optical absorption in the blue-green spectral range and demonstrated visible light induced hydrophilicity. Over the past decades, the doping of metals, especially transition metals into TiO2 have been widely investigated, which demonstrated to be a versatile strategy to enhance the wettability and the photocatalytic performance on the degradation of organic pollutants under visible light irradiation. In recent years, various transition metals have been reported to shift the absorption threshold of TiO2 into the visible region and endow TiO2 with improved photoinduce hydrophilicity and photocatalytic activity under visible light illumination.21,205,210−212 For example, Hoffmann et al. systematically investigated the effect of transition metals dopants on the

3.3. Electrowetting-Induced Superhydrophilicity

Electrowetting proved to be a versatile approach to reduce the apparent contact angle of partially wetting liquids on solid substrates by several tens of degrees using an externally applied voltage.217 Electrocapillarity, on the basis of electrowetting, was first described in detail in 1875 by Gabriel Lippmann,218 who won the Nobel prize in 1908. The electrowetting effect can be defined as the change in solid-electrolyte contact angle arising from the applied potential difference between the solid and the electrolyte. The electrostatic forces resulting from the applied electric field induced the contact angle reduction.217 Usually, microarc-oxidized TiO2 surfaces possess hydrophobicity. A variety of methods, such as thermal annealing, chemical treatment, or UV illumination, have been developed to fabricate hydrophilic microarc-oxidized TiO2. In order to enhance surface hydrophilicity, an electric polarization process was proposed by Nagai et al. to construct microarc-oxidized TiO2 coatings with durable hydrophilicity using the electrowetting effect.219 Although the electric polarization treatment could not alter the surface roughness, this approach resulted in surface electric fields and produced surface charges. A capacitor was formed at the interface between TiO2 surfaces and deionized water, partially dissociated into H3O+ and OH− at room temperature. Owing to the electric polarization, the electrostatic forces decreased the surface energy and the contact angle, resulting in the durable wettability. 3.4. Structure-Induced Superhydrophilicity

UV or visible light irradiation is an effective approach for the construction of superhydrophilic TiO2 surfaces. However, from a 10053


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reported superhydrophilic TiO2 thin films through the multilayer assembly of TiO2 nanoparticles and PEG.228 Without the use of UV irradiation, the porous TiO2 films showed superhydrophilicity. This can be ascribed to the porosity effect and surface hydroxyl, which played an important role in controlling the interaction between the liquid and film. Huang et al. prepared superhydrophilic porous TiO2 films without UV irradiation through phase separation using diethanolamine and acetylacetone as stabilizing agents.229 The pore size, pore connectivity, and gel skeleton can be controlled by changing the experimental parameters. Nair et al. reported the fabrication a transparent and superhydrophilic TiO2 coating consisting of rice-shaped nano/ mesostructures on glass substrates by electrospinning.230 The superhydrophilicity of TiO2 films increased with the increase of the TiO2 thickness, which can be attributed to the quick water chemisorption to the inner porous and randomly oriented layers. Using the self-organized linear polyethylenimine layer as a biomimetic template, Jin et al. reported the bioinspired synthesis of continuous TiO2 coatings with controlled and well-defined nanofiber network structures.231 The polyethylenimine layers served as simultaneously soft templates and catalysts for the hydrolytic condensation of water-soluble titanium bislactate. Without UV irradiation, the resultant TiO2 coating presented the superhydrophilic property with a water contact angle of about 0°. It was proposed that water adsorbed strongly on the nanofiberbased film with the enhanced capillary effect, resulting in the formation of superhydrophilic surfaces without UV irradiation. The wettability could be tunable between hydrophobicity and hydrophilicity through the ODP modification and photocatalysis decomposition under UV irradiation, respectively. Fu et al. fabricated superhydrophilic surfaces with the micronanoscale hierarchical structures using SDBS modified TiO2 nanoparticles.232 These films exhibited superhydrophilicity without UV irradiation, arising from the high surface roughness. This work opened an avenue to construct functional TiO2 for promising applications in self-cleaning and antifogging without UV irradiation. Funakoshi et al. fabricated superhydrophilic TiO2 thin films on glass substrate surfaces through the titanium alkoxide hydrolysis.233 The hydrophilicity of TiO2 films was dependent on the duration of tetraethyl orthotitanate hydrolysis. Water contact angles decreased with increasing durations of the tetraethyl orthotitanate hydrolysis reaction. When the hydrolysis reaction duration was longer than 60 min, TiO2 thin films presented superhydrophilicity with a water contact angle lower than 1°. Tang et al. reported hierarchical raspberry-like metalion-doped TiO2 hollow spheres using a spin-coating method.234 In the absence of UV irradiation, the raspberry-like TiO2 hollow spheres exhibited superhydrophilicity for more than 30 days, showing the long-term stability. This can be ascribed to the high surface roughness and high surface porosity of hierarchical raspberry-like TiO2 hollow spheres. The surface morphology TiO2 hollow spheres can be controlled through the adjustment of the molar ratio of Co2+ ions to Zn2+ ions. With the increase of the surface roughness, the hydrophilicity was enhanced. Using a polystyrene colloidal monolayer as a template, Koshizaki et al. fabricated hexagonal-close-packed, hierarchical amorphous TiO2 nanocolumn arrays by pulsed laser deposition.235 Each nanocolumn consisted of radiation-shaped nanobranches emanating from the center (Figure 13). Without UV irradiation, these TiO2 nanocolumn arrays showed superamphiphilicity with a contact angle of 0° for both water and rapeseed oil. This can be attrubuted to the pulsed laser deposition process and rough surface structures. Furthermore,

practical point of view, it is desirable to fabricate TiO2 surfaces possessing persistent superhydrophilicity without the need of external stimuli. Although the chemical composition of a solid surface affects its free energy and thus wettability, the surface geometrical structure is also an important factor on determining its wettability.220 On the one hand, TiO2 nanoparticles have a size-dependent hydrophilicity, which can be quantitatively evaluated by the reorganization energy of the interfacial charge recombination and the density of the surface hydroxyl group.221 On the other hand, it has been demonstrated that micronanoscale hierarchical structures and porous structures on the films can greatly improve surface roughness and further enhance the surface wettability.48,222−224 In the past few years, a number of efforts were devoted to designing optimal surface structural geometries to achieve TiO2 superhydrophilicity without the need of light activation. In nature, the trichome also called plant hair plays an important role in the control of plant surface wettability. Inspired by the plant trichome, Gu et al. prepared TiO2 films with trichome-like structures by the electrospinning sol−gel technique.225 The resultant films presented stable superamphilicity and self-cleaning even after the long-term dark storage. This pioneering work provides an avenue to achieve stable superamphilicity of TiO2 without light irradiation, which is important for its practical application. Inspired by the multiscale structures observed in biological systems, Mao et al. fabricated self-similar porous TiO2 surfaces using a sol−gel method on glass substrates (Figure 12).226 The bioinspired TiO2 nanostructure possessed

Figure 12. Selected images showing the spreading of a 0.5 μL water droplet at the early stages after contacting with (a) a self-similar porous TiO2 surface and (b) an uncoated glass substrate.226 Reprinted with permission from ref 226. Copyright 2010 American Institute of Physics.

stable superhydrophilicity without the need of UV or visible light irradiation. Water droplets penetrated into the porous TiO2 surface within a couple hundred milliseconds. Zeng et al. prepared transparent nanosized crack-free TiO2 thin films with good adhesion to glass substrates through a self-assembly approach. This method consisted of the synthesis of TiO2 colloidal nanoparticles, adsorptive self-assembly in the colloidal stock, and postdeposition heat-treatment.227 The resultant films exhibited superhydrophilicity with a water contact angle of about 0° and still retained at about 15° even after the dark storage for a week. Furthermore, these self-assembled TiO2 films presented high transmittance and photocatalytic activity. Amal et al. 10054


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adjusted by altering the experimental parameters including time and temperature. The sandwiched TiO2 film presented superhydrophilicity without UV irradiation, which can retain for at least 20 days. This specific wettability can be ascribed to rough surface structures and hydrothermal process-produced oxygen defects or dangling bonds. Fujishima et al. fabricated core−shell-like TiO2−SiO2 coatings using a simple electrostatic attraction method (Figure 14).241

Figure 13. SEM images of TiO2 nanocolumn arrays fabricated by pulsed laser deposition using a polystyrene colloidal monolayer as the substrate. (a and b) Low-magnification SEM images observed from the top and side. (c and d) High-resolution SEM images observed from the side. Panel d is an expanded image of panel c.235 Reprinted with permission from ref 235. Copyright 2008 American Chemical Society.

the amorphous TiO2 nanocolumn array exhibited enhanced photocatalytic activity than an anatase nanocolumn array owing to its large surface area and special microstructures. This proposed synthesis strategy was also versatile to prepare SnO2, Fe2O3, and carbon nanocolumn arrays, which can be transferred to almost any substrates. The combination of photocatalysis and superamphiphilicity endowed the surface with promising applications in self-cleaning. Utilizing the above synthesis strategy, a hierarchically ordered TiO2 hemispherical particle array with hexagonal-nonclose-packed tops was also prepared by Koshizaki et al.236 This hierarchical TiO2 particle array exhibited stable superhydrophilicity without further UV irradiation even for about half a year in air, originating from oxygen defects or vacancies on the TiO2 nanoparticle surface and the enhanced roughness of the hierarchical particle arrays. Pulsed laser deposition was developed by Bassi et al. for the anisotropic assembly of TiO2 nanoparticles, resulting in the growth of vertically oriented and columnar-like structures with a high degree of porosity. Capillary forces arising from surface wettability with different solvents and subsequent drying induced bundling of columns and resulted in a micrometer-size patterning with uniform islands separated by mesoporous channels. Without UV irradiation, the resultant TiO2 surface with a multiscale morphology exhibited superhydrophilicity, owing to the interaction between existing surface Ti3+ sites and the hierarchical morphology. Electrochemical anodization is a versatile approach for the construction of highly ordered TiO2 nanotube or nanopore arrays with a large internal surface area.237,238 These array surfaces usually exhibited superhydrophilicity arising from their high surface energy and strong capillary adsorption effect. Using a hydrothermal treatment with aqueous urea, Zhou et al. fabricated perpendicular TiO2 nanosheet films on a Ti sheet.239 Without UV irradiation, the resulting anatase TiO2 nanosheet films exhibited superhydrophilicity, which can be attributed to the edges of perpendicular nanosheets containing a large number of defects or dangling bonds. The hydrothermal synthesis strategy was also developed by Wu et al. to prepare a c-axis highly oriented sandwiched film composed of single-crystalline rutile TiO2.240 The length and density of TiO2 nanoarrays could be

Figure 14. SEM images of (a) SiO2 particle coating; (b and c) TiO2− SiO2 particle coatings prepared using TiO2 colloid solutions with different concentrations, (b) 1 mg/mL; (c) 100 mg/mL; and (d) crosssection of the sample shown in panel c. The bars in panel a−c are 500 nm; that in panel d is 200 nm.241 Reprinted with permission from ref 241. Copyright 2005 American Chemical Society.

First, the single-layered SiO2 coating was deposited on polyelectrolyte-modified glass substrates through electrostatic attraction. Then, TiO2 nanoparticles were coated on SiO2 surfaces through the same electrostatic attraction approach. Finally, the polymer was removed by calcination at 500 °C. Without UV irradiation, water droplets can spread completely on the coatings with contact angles of about 1°, exhibiting intrinsic superhydrophilicity. The improved surface wettability can be attributed to the combination of the large surface roughness and the large surface energy of the particle coating. These coatings presented self-cleaning and antireflection properties simultaneously. Although TiO2 nanoparticles possess the high refractive index, TiO2−SiO2 coatings exhibited maximum transmittances of 99%. Tang et al. reported TiO2−SiO2 composite thin films by radio frequency magnetron sputtering from a composite target. Without UV irradiation, the obtained composite thin films showed superhydrophilicity, arising from the enhanced acidity at TiO2−SiO2 interfaces.242 The heat treatment promoted the thermal diffusion of Si4+ or Ti4+ cations within TiO2 or SiO2 hosts. In order to obtain persistent superhydrophilicity without UV irradiation, Houmard et al. fabricated a series of TiO2−SiO2 composite films using a sol−gel approach.243−248 The inherent superhydrophilicity of TiO2−SiO2 films can retain several weeks in ambient aging conditions. It was demonstrated that the granular interface effect, surface physicochemical, and structural properties affected the surface wettability of TiO2−SiO2 films. Cui et al. prepared superhydrophilic surfaces on copper substrates by the layer-by-layer self-assembly and liquid phase deposition of TiO2 thin films.249 After coating with a TiO2 thin film, the surface wettability of copper was changed from hydrophobicity to superhydrophilicity. The resultant super10055


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superhydrophilic surfaces, which is important for their practical applications in antifogging, self-cleaning, etc. Utilizing the supersonic aerosol deposition, Yoon et al. reported the thermo-induced superhydrophilicity of nanostructured TiO2 films.261 The fresh TiO2 films showed modest hydrophilicity with a water contact angle of 45°. With the increase of annealing temperature, the water contact angle was decreased (Figure 15). After calcination at 500 °C, water droplets

hydrophilic surfaces could retain their wettability even after the boiling treatment. Mesoporous TiO2 materials with pore size between 2 and 50 nm have a wide domain of applications in catalysis, sensors, membrane, optics, separation techniques, and smart coatings, associated with their high specific surface areas. The design of mesoporous TiO2 films with desired surface wettability should extend their practical applications. Utilizing the evaporationinduced self-assembly method, Fu et al. fabricated a series of mesoporous TiO2 and its composite films using different template agents.250−253 They found the resultant mesoporous TiO2 and its composite films exhibited intrinsic hydrophilicity in the absence of light irradiation.252,253 This can be attributed to the rough surfaces with mesoporous structures. Utilizing the similar method, Wang et al. prepared mesoporous TiO2 thin films using the triblock copolymer as a template.254 The obtained film exhibited hydrophilicity without UV light illumination. The crystallite size and porosity affected the surface hydrophilicity of mesoporous TiO2 thin films. Boxall et al. fabricated transparent mesoporous TiO2 thin films on quartz using the hydrothermal reverse micelle route.255 Using a continuous measurement technique, the time dependence of the photoinduced superhydrophilicity of the TiO2 film was evaluated through the spreading of a sessile water drop upon UV illumination. It was found that mesoporous TiO2 films exhibited a photoinduced stick−slip behavior. The thermodynamic driving force for this behavior can be ascribed to the departure of the system from capillary equilibrium as the surface Ti−OH group concentration increased and the equilibrium contact angle of the drop decreased with the increase of irradiation time. Usually, the superhydrophilic surface was defined using the contact angle value of a water droplet on the solid surface. Recently, capillary rise measurements were developed by Buie et al. to characterize superhydrophilic surfaces in terms of capillary pressure and spreading speed.256 They fabricated superhydrophilic surfaces with hierarchical micro/nanoscale pores through the combination of breakdown anodization and electrophoretic deposition. During the electrophoretic deposition, TiO2 nanoparticles were used to increase the surface energy and produce nanoporous structures, while the breakdown anodization was employed to form microporous structures. It was found that microporous structures and nanoporous structures have an effect on the transport properties and local wettability, respectively.

Figure 15. Relationship between the water contact angle and annealing temperature, without UV irradiation.261 Reprinted with permission from ref 261. Copyright 2013 American Chemical Society.

completely spread and infiltrated the films, exhibiting the superhydrophilicity without UV irradiation. In contrast to UVinduced temporary superhydrophilicity, thermo-induced superhydrophilicity appears to be permanent. Even after three months, no apparent change was found for the water contact angle. This can be ascribed to the combined effects of the change in the surface roughness, the content of surface hydroxyl groups, and crystal structure. Fu et al. fabricated transparent TiO2 films on glass substrates through the evaporation-induced assembly approach.262,263 With the increase of calcination temperatures, a noticeable decrease in the water contact angle was found. Thermo-induced superhydrophilicity can be attributed to the increase of the surface roughness upon the increase of calcination temperatures, which has been demonstrated by the AFM studies. These obtained transparent and uniform nanocrystalline TiO2 films have promising applications in self-cleaning and antifogging. Meng et al. prepared TiO2 films with thermo-induced hydrophilicity on silicon and quartz substrates by radio frequency magnetron sputtering.264 It was found that the crystal structure played an important role in the surface wettability of TiO2 films. When the annealing temperature was about 900 °C, the mixture of anatase and rutile induced optimal hydrophilicity. Du et al. demonstrated that thermo-induced superhydrophilicity can be enhanced by doping Mo(VI) ions in TiO2 films.265 The introduction of Mo ions resulted in the increase of defective sites on TiO2 surfaces. Because MoO6 did not possess hydrophilicity, the hydrophilicity decreased when the doping content exceeded a critical level (0.75 wt %). Farbod et al. fabricated superhydrophilic TiO2−SnO2 nanocomposite thin films by a sol−gel dip-coating technique. After calcination at 500 °C, the resultant film showed superhydrophilicity. Hashimoto et al. reported the hydrophilicity of TiO2 thin films can be improved through the high-temperature treatment.266 The low temperature anneal (e.g., 100 and 150 °C) could not produce obvious changes of water contact angles. However, after higher

3.5. Thermo-Induced Superhydrophilicity

Recent studies demonstrated that the thermal treatment can enhance the surface wettability of TiO2 films. Thermo-induced superhydrophilicity can be explained in the following considerations: (a) The cleansing effect. Thermal treatment removed the superficial organic contaminants, which facilitated the adsorption of water molecules on TiO2 films.257 (b) The crystal structure transition. It has been demonstrated that the surface wettability of TiO2 films is associated with the crystalline phase.258 (c) The change of surface chemical compositions, such as surface hydroxyl content, Ti3+ defect sites, and oxygen vacancy sites. It was found that a special type of surface-active sites, such as Ti3+ defect sites and oxygen vacancy sites, can be created on the TiO2(110) surface by thermal annealing to high temperatures.259 The hydrophilicity can be enhanced through the increase of the Ti3+ concentration on TiO2 surfaces.260 (d) The increase of surface roughness. Thermal treatment was proved to be an effective approach for the fabrication of permanently 10056


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nonfluorinated coating materials were usually used to decrease the surface free energy of the substrates. (b) The design of special surface structures. Recent research work demonstrated that a superhydrophobic behavior can be achieved on a intrinsically hydrophilic surface through the construction of overhanging microtextures.182 Now, without surface modification, superhydrophobic TiO2 can also be fabricated through the rational design of surface structures. The adhesive behavior of a water droplet on a superhydrophobic surface is usually governed by the state of the three-phase-contact line (such as shape, length, continuity of contact, and amount of contact), van der Waals forces, and capillary forces.144−154 For example, a discontinuous three-phase contact line could induce a low water contact angle hysteresis. Water droplets on this superhydrophobic surface can roll freely in all directions, showing low adhesive superhydrophobicity similar to lotus leaves found in nature. However, a continuous three-phase contact line results in a serious water contact angle hysteresis. Water droplets with spherical shape can pin on this superhydrophobic surface even when it was turned upside down, exhibiting high adhesive superhydrophobicity similar to gecko feet found in nature.40 In this section, we will discuss superhydrophobic TiO2 materials, which can be classified into the following three categories: (a) low-adhesive superhydrophobic TiO2, (b) high-adhesive superhydrophobic TiO2, and (c) superhydrophobic TiO2 surfaces with switchable water adhesion.

temperature treatments, apparent decreases of water contact angles were observed on TiO2 films. This work provided supporting evidence for the clarification that surface defective sites played crucial roles in the photoinduced surface wettability conversion on TiO2 thin films.

4. SUPERHYDROPHOBIC TITANIUM DIOXIDE MATERIALS In nature, after billions of years of evolution, some biological surfaces exhibit fascinating superhydrophobicity due to the cooperation between surface multiscale structures and surface chemical compositions, such as lotus leaves (Figure 16), rice

4.1. Low-Adhesive Superhydrophobicity

In nature, there are many biological surfaces possessing low adhesive superhydrophobicity. Among the wide variety of low adhesive superhydrophobic biomaterials, the lotus leaf is one of most promising. Water droplets falling onto the lotus leaf bead up, roll freely in all directions and then pick up dirt particles, showing the self-cleaning effect (Figure 16). This can be attributed to the cooperation of surface hydrophobic epicuticular wax and surface multiscale structures with randomly distributed micropapillae covered by branch-like nanostructures. Inspired by the lotus leaf, many different synthesis strategies have been developed to fabricate superhydrophobic TiO2 surfaces with low contact angle hysteresis by creating appropriate surface geometrical structures and other specific components. Inspired by the superhydrophobic lotus leaf, in 2000, Hashimoto et al. fabricated transparent superhydrophobic TiO2 thin films after FAS modification (Figure 17).310 Even after the UV irradiation with the intensity of 1.7 mW/cm2 for 800 h, the water contact angle of TiO2 thin films containing 2 wt % TiO2 could maintain more than 140°. The transparency of the films decreased with the increase of TiO2 concentration, which can be attributed to the size difference of the starting materials. This is the first report of TiO2 thin films simultaneously possessing superhydrophobicity, transparency, long lifetime, and self-cleaning, which have promising applications in industrial fields. Lyons et al. reported a template lamination method for the fabrication of superhydrophobic TiO2−HDPE nanocomposite surfaces, which possess three levels of hierarchical roughness.314 Without surface chemical modification, the as-prepared surfaces exhibited superhydrophobicity with a low sliding angle and selfcleaning. In addition to the superhydrophobicity, TiO2−HDPE nanocomposite surfaces exhibits a UV-thermal induced reversible wettability. Utilizing a codeposition technique, the superhydrophobic nanocomposite coating of TiO2 and PTFE was fabricated by Yamashita et al. on a structured substrate.282 Different from the photoinduced superhydrophilicity of TiO2,

57

Figure 16. (a) Large-area SEM image of the lotus leaf surface. Every epidermal cell forms a papilla and has a dense layer of epicuticular waxes superimposed on it. (b) Enlarged view of a single papilla from (a). Reprinted with permission from ref 57. Copyright 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) SEM image of 3D epicuticular wax tubules on lotus leaf surfaces, which create nanostructures.68 Reprinted with permission from ref 68. Copyright 2011 Elsevier. (d) Water droplets roll easily across the lotus leaf surface and pick up dirt particles, demonstrating the self-cleaning effect.

leaves, butterfly wings, water strider legs, silver ragwort leaves, red rose petals, gecko feet, and peanut leaves.30−32,35,39,40,57,267,268 Wettability of solid surfaces with liquids is usually governed by their surface geometrical structures and surface free energy. In the past few decades, many different synthesis strategies have been developed to prepare bioinspired superhydrophobic surfaces through the combination of surface roughness and surface compositions. Recently, a number of detailed reviews covering fabrication, characterization, and application of bioinspired superhydrophobic surfaces have been published.47−77,269−275 Research on superhydrophobic surfaces has not only basic research interest but practical applications in the fields of industry, agriculture, and daily life.6,7,77 As discussed in the preceding sections, TiO2 possesses special photoinduced properties: photocatalysis and photoinduced superhydrophilicity. Recently, owing to their promising applications, a wide variety of superhydrophobic TiO2-based materials have been constructed by using different synthesis strategies (Table 1). These strategies can be categorized following two principal approaches: (a) The surface modification. Surface chemical compositions play an important role in generating the desired surface wettability. Therefore, in order to fabricate superhydrophobic TiO2, hydrophobic fluorinated or 10057


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Table 1. Table of Superhydrophobic Titanium Dioxide Materials Fabricated using Different Approaches method ALD AVO anodization APPJ casting casting codeposition CVD electrochemical/SA electrochemical/SA electrochemical/SA electrodeposition electrophoretic deposition electrospinning electrospinning electrospinning FRP hydrothermal hydrothermal hydrothermal impregnation LFS LFS liquid phase photocatalysis plasma etching SA/dry etching sol−gel sol−gel/hydrothermal solvothermal solvothermal spin-coating spin coating spraying spraying spraying template lamination

modifier

perfluorosilane FPU, PMPS, APS VTEOS, PS PTFE PFAMA POTS POTS POTS-NC FAS POTS FAS PS−PVC POTS PS PFTS stearic acid siloxanes

TMPSi TCPEOS ODP PFTS perfluorophosphate surfactant FAS NH4F C16 FAS C12 ODT PFOA HDPE

TiO2 structure nanoparticles tree-like nanoarrays nanotubes tree-like nanoparticles nanoparticles nanoparticles nanospheres nanotubes nanotubes spongelike nanoparticles nanotubes nanoparticles nanoparticles nanofibers core−shell nanorods nanospheres cactus-like nanoparticles nanoparticles nanoparticles nanoparticles nanoparticles wedgelike nanocone arrays nanoparticles nanoparticles nanoparticles hollow spheres nanoparticles nanoparticles nanoparticles nanoparticles nanoparticles nanoparticles

additional function low adhesion structure-induced tunable wettability tunable and switchable water adhesion tunable water adhesion low adhesion self-cleaning, UV induced reversible wettability self-cleaning structure-induced controllable water adhesion surface composition-induced reversible wettability and adhesion surface composition-induced controllable water adhesion high adhesion low adhesion low adhesion structure-induced water adhesion high adhesion high adhesion, light harvesting tunable water adhesion low adhesion self-cleaning heat-induced reversible wettability structure-induced controllable water adhesion high adhesion low adhesion structure-induced tunable hydrophobicity low adhesion high adhesion high adsorption capacity tunable wettability low adhesion self-cleaning low adhesion reversible adhesion and anticorrosion low adhesion, self-cleaning, UV induced reversible wettability

refs 276 277 278 279 280 281 282 283 237 284 285 286 287 288 289 290 291 292 293 294 295 296−298 299,300 301 302 303 304 305 306 307 308 309 310 311 312 313 314

and self-cleaning. Using an arrangement of TiO2 nanospheres followed by the surface fluorination treatment, Hsieh et al. prepared superhydrophobic TiO2 surfaces with a low sliding angle through the CVD approach. 283 After the PFOA modification, superhydrophobic TiO2/polystyrene surfaces have been fabricated by Zhang et al. using a spraying process.313 The surface wettability of the nanocomposites coating is dependent on the drying temperature and the ratio of TiO2 nanoparticles to polystyrene. For example, a superhydrophobic surface with the contact angle of about 166° can be obtained at 180 °C, when the ratio of TiO2 nanoparticles to polystyrene was 1:1. When the ratio was increased, the water contact angle can reach 157° at 100 °C. These superhydrophobic coatings were expected to have potential applications in the field of selfcleaning. C16 has long alkyl chains, which can be used as the hydrophobic modifier.315 Holtzinger et al. reported the fabrication of superhydrophobic TiO2 coatings using the nonfluorinated C16 as the modifier.309 TiO2 sol−gel films were structured by PS nanosphere lithography. In optimized conditions, the water contact angle can reach 160° with the sliding angle 1°. Inspired by the superhydrophobic biological surface, Lindén et al. reported the synthesis of superhydrophobic

Figure 17. SEM and water contact angle images of the transparent superhydrophobic films containing 20% TiO2.310 Reprinted with permission from ref 310. Copyright 2000 American Chemical Society.

even after five cycles of adhesions of oleic acid and UV light irradiation, the resultant TiO2−PTFE coating can restore its superhydrophobic state, exhibiting stable superhydrophobicity 10058


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Figure 18. Schematic procedure of the formation of PSAA/TiO2 hybrid hollow spheres.308 Reprinted with permission from ref 308. Copyright 2013 Royal Society of Chemistry.

philic to hydrophobic, while TiO2 presented the reverse trend. Utilizing the electrospinning approach, Asmatulu et al. fabricated superhydrophobic electrospun PS−PVC nanocomposite fibers.289 TiO2 nanoparticles and graphene nanoflakes were incorporated into the electrospun polymer fibers. It was found that the experimental conditions, such as nanoscale inclusions, concentrations, and electrospun fiber diameters strongly affected the surface superhydrophobicity. The resultant TiO2- and graphene-based superhydrophobic photoelectrodes were expected to enhance the efficiency of dye-sensitized solar cells as self-cleaning and anti-icing materials. Eder et al. reported CNTsTiO2 hybrid materials using benzyl alcohol as a surfactant, which enabled TiO2 to interact with hydrophobic CNTs without covalent functionalization.330 Furthermore, the morphology and structure of the resultant CNTs-TiO2 composites can be controlled by the initial coating thickness. In the past decades, hollow spheres have attracted considerable attention owing to their unique physical characteristics and promising applications in the field of catalysis, sensors, and batteries. Utilizing the Kirkendall effect, Wu et al. fabricated PSAA/TiO2 polymeric/inorganic bilayer hybrid hollow spheres (Figure 18).308 The water contact angle of the as-obtained PSAA/TiO2 hollow spheres is about 85°. However, after the treatment in ethanol at 180 °C for 12 h, it converted into superhydrophobicity with a contact angle of 156°, which can be attributed to the diffusion of hydrophobic polymer chains from the inner layer to the outer layer of hollow spheres during the solvothermal process. In contrast, after the hydrothermal treatment, it became superhydrophilic with a contact angle of 0°. It has been demonstrated that this method is also versatile for the fabrication other bilayer hybrid hollow spheres, such as PSAA/ZrO2, PSAA/Nb2O5, and PSAA/Ta2O5. Using the photocatalytic property of TiO2, Tomovska et al. reported the application of UV or visible light for the initiation of surface reactions on TiO2 nanoparticles with TCPEOS.302 It was found that chloropropyl functionalities were grafted on TiO2 nanoparticle surfaces, followed by intensive silylation, resulting in superhydrophobicity. Ardizzone et al. reported the fabrication of superhydrophobic siloxane-TiO2 hybrid nanocomposites using a mild impregnation method.295 The resultant nanocomposites showed complete buoyancy over water and selfcleaning properties. It was demonstrated that siloxane groups covalently attached to the TiO2 surface, which could serve as powerful linkers for further functionalization. Using isopropanol as the solvent and inorganic NH4F as the hydrophobic modifier, Zhang et al. fabricated a superhydrophobic mesocellular foamsTiO2 composite through a solvothermal method.307 Moreover, these superhydrophobic mesoporous materials exhibited highadsorption capacity and enhanced photocatalytic degradation for Rhodamine B. Utilizing self-assembly of colloidal microspheres

coating by precipitating calcium phosphate on sol−gel-derived TiO2 films under in vitro conditions.305 The formation of the calcium phosphate layer is self-organizing, and the coating can be functionalized with the perfluorophosphate surfactant to reach the superhydrophobicity. It was proposed that this methodology could be extended to the synthesis of superhydrophobic CaCO3 materials. Electrospinning is a simple but versatile method for the fabrication of continuous fibers for various materials, which has been demonstrated to be effective to generate appropriate roughness for surface superhydrophobicity.316−318 Inspired by the self-cleaning silver ragwort leaves with multiscale structures, Shiratori et al. fabricated electrospun cellulose acetate nanofibrous membranes.288 Layer-by-layer structured films were assembled on the electrospun nanofiber surface by alternating adsorption of negatively charged PAA and positively charged TiO2 nanoparticles. After modification with FAS, the surface wettability was changed from superhydrophilicity to superhydrophobicity with a low rolling angle. Combining electrospinning, CVD, and hydrothermal methods, He et al. reported the fabrication of superhydrophobic anatase TiO2 film following by the POTS modification.290 Relationships among surface microstructure, hydrophobicity, and adhesive forces were also investigated. It was found that proper hydrothermal treatment will be helpful to decrease the sliding angle and increase the selfcleaning property. Carbon-based nanomaterials, such as carbon black, activated carbon, carbon nanotubes, fullerene, and graphene for the enhancement of TiO2 photocatalysis have attracted considerable attention arising from their unique structural and electrical properties. These topics are well-covered in recent several comprehensive reviews.319−323 Although the mechanism of carbon-based dopant has not been fully understood, the introduction of carbon-based nanomaterials should facilitate the retardation of electron−hole recombination, band gap tuning/photosensitization, or provision for adsorption and active sites. Graphene is a single-atom-thick sheet of sp2hybridized carbon atoms, which has attracted a great deal of attention arising from its exceptional physicochemical properties. Experimental and theoretical studies indicated graphene possessed the inherent hydrophobic characteristic.324−327 Furthermore, through changing the relative proportion of water and acetone in the solvent, the wettability of graphene films can be tuned from superhydrophobicity to superhydrophilicity.328 Combining TiO2 with graphene should affect the original surface wettability. Zhu et al. reported graphene oxide/TiO2 hybrid films through the combination of layer-bylayer and drop-casting methods.329 Arising from the photocatalytic reduction of graphene oxide by TiO2, the graphene oxide showed UV responsive wetting conversion from hydro10059


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Figure 19. Typical biological materials with high adhesive superhydrophobicity and corresponding surface structures. (a) The peanut leaf,39 Reprinted with permission from ref 39. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) The red rose petal,35 Reprinted with permission from ref 35. Copyright 2008 American Chemical Society. (c) The gecko foot,40 Reprinted with permission from ref 40. Copyright 2012 Royal Society of Chemistry.

hydrophobicity with a water contact angle as high as 170°. ALD is a versatile and effective method to replicate and program the nanopatterns of biological surfaces.332 Recently, ALD was proved to be useful by Szilágyi et al. for the fabrication of superhydrophobic lotus/TiO2 composites by depositing TiO2 nanofilms on the lotus leaf.276 Besides the preservation of the lotus leaf’s superhydrophobicity, photocatalyticity was introduced in the composites. By changing the TiO2 film thickness, the surface wettability and photocatalyticity of the lotus/TiO2 material can be tailored.

or nanospheres, it has been demonstrated that colloidal lithography is an effective approach for the fabrication of ordered nanopatterning arrays.331 Park et al. fabricated 2D structures of highly ordered TiO2 nanocones with superhydrophobicity by combining self-assembly, dry etching, and postmodification with PFTS.304 Through controlling the surface roughness, the wettability of nanopatterned TiO2 layers can be tuned from hydrophobicity to superhydrophobicity. These TiO2 nanopatternings have promising applications in the fields of antifouling and self-cleaning. In order to improve the durability of superhydrophobic surfaces, Zhou et al. prepared superhydrophobic TiO2-based nanocomposite coatings with good mechanical properties and durability by blending room-temperature-curable fluoropolysiloxane with TiO2 nanoparticles.280 The binder consisted of FPU, PMPS, and APS. Through altering the binder ratio, the water adhesion property can be tailored. Furthermore, these superhydrophobic materials exhibited robust durability in accelerated weathering tests, good mechanical properties even when cured at ambient temperatures, and resistance to organic contaminants. Wu et al. fabricated hierarchically nanostructured rutile arrays through the acid vapor oxidation approach.277 The adjustment of the HCl concentration resulted in the tunable morphologies and tailored surface wettability from hydrophobicity to hydrophilicity. A LFS process was developed by Stepien et al. to prepare superhydrophobic TiO2-coated paperboard surfaces.296 XPS measurements indicated that, in comparison with the SiO2coated sample, there is much more carbon C1 of hydrocarbon type bonds in the TiO2-coated sample. It was proposed that the replacement of hydroxyl groups by aliphatic chains resulted in the superhydrophobicity for the TiO2-coated paperboard. Furthermore, heat treatment in the oven can significantly speed up the wettability conversion between superhydrophobicity and superhydrophilicity.297 Zhang et al. reported the fabrication of superhydrophobic tree-like rutile/anatase TiO2 using the APPJ method on quartz glass substrates.279 The formation of TiO2 crystal trees is independent of the monomer type and only associated with plasma discharge conditions and the residence time of the precursor in the plasma zone. Without surface modification, these TiO2 crystal trees exhibited super-

4.2. High-Adhesive Superhydrophobicity

In the preceding section, the progress in low adhesive superhydrophobic TiO2 materials similar to the lotus leaf has been summarized. In nature, biological surfaces with high adhesive superhydrophobicity also can be found, such as the peanut leaves, red rose petals, and gecko feet (Figure 19), arising from their special surface structures. Inspired by these natural materials, a variety of high adhesive superhydrophobic materials have been fabricated through the rational design of surface structures, exhibiting promising applications in microfluidic systems, no-loss liquid transportation, and other fields. This section will focus on the recent developments in superhydrophobic TiO2 materials with high water adhesion. Weibel et al. fabricated superhydrophobic surfaces with different liquid−solid adhesion properties using TiO2 and lower alkyl chain silane (trimethoxypropyl silane).301 The contact angle hysteresis of superhydrophobic surfaces prepared in aqueous solvent is higher than that in nonaqueous solvent. Water droplet could roll on the superhydrophobic surface fabricated in aqueous solvent but will not roll off out of the surface. It was proposed that the difference in liquid−solid adhesion is attributed to the distinction of surface hierarchical structures and surface chemical compositions. Combining sol− gel based amorphous TiO2 and the hydrothermal approach, Takahashi et al. prepared titanate nanofunnel brushes.306 After modification with fluoroalkylsilane, the brush surface becomes high adhesive superhydrophobic, which can be used for small water droplet delivery. Furthermore, the surface morphology can be tuned from nanosheets to nanofunnels by tailoring the NaOH 10060


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concentration. An electrophoretic deposition process was developed by Chen et al. to fabricate titanate nanotube films with superhydrophilicity.287 After modification with low surface energy POTS, titanate nanotube films converted to superhydrophobicity with a high adhesion to water. Through annealing or low temperature wet chemical process, the titanate film can be changed to anatase TiO2 film preserving the original surface morphology. Although rutile TiO2 is a hydrophilic material with a water contact angle of more than 70° on smooth single crystal (110) and (001),80,83 surface structures strongly affect the wetting behavior of TiO2. Now, highly hydrophobic, even superhydrophobic TiO2 surfaces have been fabricated through the construction of special surface structures. Using a hydrothermal synthesis method, Li et al. prepared rutile TiO2 nanorod films with a highly hydrophobicity.292 On the basis of the hydrophobicity of TiO2 nanorods, dandelion-like ZnO/TiO2 heterogeneous nanostructures were fabricated through a heteroepitaxial growth of ZnO nanorods selectively on the tips of TiO2 nanorods. It was found that branched ZnO structures with heteroepitaxial interfaces resulted in the efficient charge transport efficiency and improved light harvesting performance.

Figure 20. Schematic diagram of producing TiO2 nanotubes with switchable wettability and adhesion. (a) SEM image of the as-prepared aligned TiO2 nanotube array via anodization method. (b) Fluorescence microscope image of TiO2 nanotube surfaces after UV/mask treatment with high adhesion.278 Reprinted with permission from ref 278. Copyright 2009 Royal Society of Chemistry.

microwaves. It was proposed that upon annealing, the organic chains may experience realignment with the −CF3 group stretching outside the top surface, and loss of absorbed water or the hydroxyl groups on photocatalytic sites. Recently, Takahashi et al. further demonstrated the effect of heat on the water adhesion on superhydrophobic surfaces.334 After FAS modification, the titanate nanotube film exhibited high adhesive superhydrophobicity upon exposure to moisture, whereas it becomes repellent toward water after mild heating, showing switchable and reversible water adhesion. By alternating UV irradiation and acid treatment, Zhang et al. fabricated superhydrophobic Ag-TiO2−Thiol/PMMA coatings with reversible switching of water adhesion.312 Superhydrophobic composite surfaces were fully covered by the thiol layer, where the long hydrocarbon chain terminal groups of ODT orientated outside. This resulted in a high air fraction and then low adhesive superhydrophobicity. Upon UV irradiation, Ag-TiO2−Thiol/ PMMA surface became high adhesive superhydrophobic arising from the formation of hydroxyl groups. After being immersed in hydrochloric acid, the adhesion of the coating changed from the high adhesive state to the sliding state, which can be attributed to the content decrease of hydroxyl groups and the rearrangement of thiol molecules on the outmost coating in the acidic condition. The UV irradiation could not result in the photoinduced superhydrophilicity but hydrophobicity owing to the thiol grafted Ag nanoparticle covering the TiO2 surface. Electrochemical measurements showed these superhydrophobic coatings possess stable anticorrosion. 4.3.2. Surface Chemical Compositions-Induced Controllable Adhesion. The wettability of solid surfaces is usually governed by their surface chemical compositions and surface geometrical structures. Therefore, it is possible to fabricate superhydrophobic TiO2 surfaces with controllable adhesion through rationally manipulating surface free energy. Using the electrochemical and self-assembly methods, Lin et al. reported the fabrication of superhydrophobic sponge-like nanostructured TiO2 surfaces followed by the POTS-NC modification.285 The adhesion forces between 5.0 μN and 76.6 μN toward water can be controlled by altering the concentration of NC dosage. After introducing the NC, competition will occurs between the NC and POTS molecules for the hydroxyls on hydroxylated TiO2

4.3. Superhydrophobic Surfaces with Switchable Water Adhesion

In nature, reversible solid−solid adhesion can be found. For example, gecko is an amazing animal and has evolved one of the most versatile and reversible solid−solid adhesions, which has a unique ability to cling to and detach from walls.333 However, switchable liquid−solid adhesion is extremely rare in nature. Recently, smart superhydrophobic surfaces with switchable water adhesion have attracted much attention owing to their more promising applications in comparison with the single low adhesive or high adhesive superhydrophobic surfaces.65 These smart superhydrophobic surfaces have significant application in liquid transportation, microdevices, and microfluidic channels.59 It has been demonstrated that external stimuli, surface composition, or surface morphology can affect the droplet mobility on superhydrophobic surfaces. In this section, we will mainly focus on the superhydrophobic TiO2 surfaces with switchable water adhesion through the light stimuli and the regulation of surface chemical composition and surface structures. 4.3.1. Light-Induced Switchable Adhesion. Light is one of the most important external stimuli. As a typical photosensitive inorganic oxide, TiO2 exhibits photocatalytic and photoinduced superamphiphilic properties.4 By taking advantage of special photoinduced properties, superhydrophobic TiO2 surfaces with switchable water adhesion have been fabricated using different synthesis strategies. Utilizing the anodization approach followed by the perfluorosilane modification, Zhou et al. reported the fabrication of nanotube films (Figure 20).278 The water droplet on superhydrophobic TiO2 surfaces can be switched between sliding and sticky superhydrophobicity by masked UV illumination and heat annealing. Upon the UV irradiation with a mask, patterned wettability with hydrophilic/superhydrophobic regions was formed on the superhydrophobic TiO2 nanotube film. For the photoinduced hydrophilic regions, the water adhesion was increased. In contrast, the surrounding superhydrophobic regions prevented the spread of water droplets and preserved superhydrophobicity. Furthermore, the recovery was very fast upon heating at a relatively high temperature or the use of 10061


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surfaces. The combination of the disruption of the densely packed hydrophobic POTS molecules and hydrogen bonding between the hydrophilic NC nitro groups and the hydroxyl groups at the solid/liquid interfaces should be responsible for the increase of water adhesion on superhydrophobic sponge-like TiO2 films modified with POTS and NC. It has been demonstrated that this method also can be used to fabricate in situ surface-modification-induced superhydrophobic TiO2 nanotube arrays with reversible wettability and water adhesion.284 Through the ink printing and erasing, the water adhesion of superhydrophobic TiO2 nanotube arrays could be reversibly regulated between sticky and sliding state. For the ink-patterned area, the water droplet can pin to the TiO2 surface. However, after removal of the alcohol-based ink layer by rinsing in methanol solution, the adhesion of TiO2 arrays was changed from high adhesive to low adhesive superhydrophobic state. Even after many cycles, this reversible adhesion switching can be well retained. 4.3.3. Surface Structures-Induced Tunable Adhesion. In the preceding section, we have discussed surface chemical compositions-induced controllable adhesion of superhydrophobic TiO2 surfaces. The surface geometry also plays a fundamental role in the surface wettability of solid substrates. This section will focus on the effect of surface structures on the water adhesion of superhydrophobic TiO2 surfaces. On the basis of the basic principles of roughness-enhanced hydrophobicity and capillary-induced adhesion, Gao et al. reported the tunable water adhesion on superhydrophobic TiO2 porous nanostructures with different morphologies.237 Three superhydrophobic nanostructure models were proposed through the construction of a nanopore array, a nanotube array, and a nanovesuvianite structure using the electrochemical anodization method (Figure 21). The surface adhesive forces could be manipulated through the solid−liquid contact ways and the fractions of air pockets in open and sealed systems. Furthermore, the adhesion of superhydrophobic TiO2 nanotube arrays was dependent on the nanotube diameter and length, which can be attributed to the negative pressure changes resulting from the volume changes of air sealed in the nanotubes. Ou et al. fabricated superhydrophobic TiO2 through the hydrothermal approach after PFTS modification.293 By controlling the hydrothermal treating time, surface structures-induced tunable water adhesion could be achieved between Wenzel state and Cassie state. Teisala et al. found that changes in the hierarchical structure of TiO2 nanoparticle surfaces fabricated using the LFS approach on board and paper substrates could result in the variation of water and water−ethanol adhesion between the high- and low-adhesive superhydrophobic states.299,300

Figure 21. Schematic illustration of three types of superhydrophobic porous nanostructure models with water adhesive forces ranging from high to low. (a) Capillary adhesion arises when a water droplet sitting on the tube nozzle is gradually drawn upward because the convex air/liquid interface produces an inward pressure ΔP. (b) Superhydrophobic NPA with high adhesion. (c) Superhydrophobic NTA with controllable adhesion. (d) Superhydrophobic NVS with extremely low adhesion.237 Reprinted with permission from ref 237. Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

lichens (Cladonia chlorophaea), etc. (Figure 22).37,45,340,341 Inspired by these biomaterials, a variety of superhydrophilic− superhydrophobic patterns have been fabricated using different synthesis approaches on different substrates.335−337,342−344 In the preceding sections, superhydrophilic TiO2 materials and superhydrophobic TiO2 materials have been summarized separately. In this section, we will focus on the recent progress in the fabrication of TiO2-based superhydrophilic−superhydrophobic patterns. A more detailed discussion on the emerging application of TiO2-based functional patterns combining both wetting properties of superhydrophilicity and superhydrophobicity will be presented in Section 8.

5. SUPERHYDROPHILIC−SUPERHYDROPHOBIC PATTERNS Superhydrophilic or superhydrophobic surfaces are ubiquitous in nature, which provide important inspiration for scientists and engineers to design and to fabricate functional surfaces with special wettability. The combination of two extreme wetting states of superhydrophilicity and superhydrophobicity on the same surface has some interesting applications in water collection, liquid transportation, microfluidics, printing, and biomedical fields.335−339 In nature, some biomaterials have evolved different optimized structures with superhydrophilic− superhydrophobic patterns to survive, such as the tenebrionid beetle (Stenocara sp.), water fern (Salvinia molesta), cups of

5.1. Photomask-Induced Wetting Patterns

In recent years, Fujishima et al. reported a series of pioneering works on the TiO2-based superhydrophilic−superhydrophobic patterns and their applications in offset printing, water condensation, microreactor arrays, etc. Through roughening 10062


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Figure 22. (a) Superhydrophilic peaks and superhydrophobic troughs on the surface of an adult female beetle (Stenocara sp.) living in the arid Namib Desert. (b) SEM image of the textured surface of the depressed areas.37 Reprinted with permission from ref 37. Copyright 2001 Nature Publishing Group. (c) Morphology of the floating water fern Salvinia molesta possessing unique surface architecture composed of complex hydrophobic eggbeater-shaped hairs with hydrophilic terminal cells.45 Reprinted with permission from ref 45. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) A water droplet fastened by superhydrophilic microspots on the rim of the superhydrophobic cuplike structure of the Cladonia chlorophaea lichen.340 Reprinted with permission from ref 340. Copyright 2011 Springer-Verlag. Scale bars (a) 10 mm; (b) 10 μm; (c) 5 mm; (d) 1 mm.

Figure 23. SEM image of a superhydrophobic-superhydrophilic pattern prepared using a photomask (line/space ratio: 1:1). Bright stripes in the image are superhydrophobic, and dark stripes are superhydrophilic.345 Reprinted with permission from ref 345. Copyright 2007 American Chemical Society. (b) Optical micrographs of the ODS-modified CuO pattern on the TiO2 thin film obtained by the process described in the text with a photomask of 500-μm2 blocked areas.347 Reprinted with permission from ref 347. Copyright 2008 Elsevier. (c) Optical microscope photograph of water droplet on the superhydrophobicsuperhydrophilic surface prepared by the irradiation of UV light through a photomask.348 Reprinted with permission from ref 348. Copyright 2000 American Chemical Society. (d) Fluorescence microscope image of the superhydrophilic−superhydrophobic TiO2 nanotube pattern. The green dot patterns were imaged through the fluorescence contrast between the UV-irradiated superhydrophilic and photomasked superhydrophobic regions.349 Reprinted with permission from ref 349. Copyright 2008 Elsevier. Scale bars a, 100 μm; b, 500 μm; d, 250 μm; d, 200 μm.

the smooth polycrystalline TiO2 film to produce a nanocolumnar morphology followed by the ODP modification, a superhydrophobic TiO2 surface with low water adhesion was fabricated.345 Under the UV illumination, superhydrophobic TiO2 surfaces were able to be converted to superhydrophilic surfaces owing to the photocatalytic decomposition of ODP. Using a photomask, superhydrophilic−superhydrophobic patterns with 50 μm wide superhydrophilic stripes were fabricated on TiO2 surfaces (Figure 23a). These patterns could guide the water condensation and evaporation of polystyrene microsphere suspensions arising from the extremely large wettability contrast between superhydrophobic and superhydrophilic areas. In order to fabricate transparent superhydrophilic−superhydrophobic TiO2 patterns, a three-step method was proposed, which consists of the fabrication of a porous TiO2 film by spin-coating on glass substrates, surface modification with FAS molecules, and UV illumination through a photomask.346 These surface-patterned films exhibited good stability under indoor conditions, which have promising applications in site-selective immobilization of biomolecules. A new approach was developed to shorten the area-selective UV irradiation time through the combination of the photocatalytic Ag nucleation on TiO2 and the electroless Cu deposition (Figure 23b).347 Using this synthesis strategy, the UV irradiation time was drastically decreased to 1 s for the nucleation step in the patterning process. Tadanaga et al. reported the fabrication of superhydrophilic− superhydrophobic patterns on the alumina film.348,350 In the first stage, flowerlike Al2O3 films were coated on soda lime glass plates through a dipping-withdrawing approach. Then a very thin TiO2 layer was coated on the flowerlike Al2O3 surface. Finally, partially hydrolyzed FAS was used as the modifier and coated as the top layer on the TiO2 layer with an underlayer of flowerlike Al2O3. Under UV irradiation with a photomask, well-defined superhydrophilic−superhydrophobic patterns were prepared (Figure

23c). It has demonstrated that the fluoroalkyl chain in FAS can be cleaved by the TiO2 photocatalytic reaction. Lin et al. fabricated superhydrophilic−superhydrophobic patterns on TiO2 nanotube structured films.349 First, the superhydrophobic TiO2 nanotube was prepared through electrochemical anodizing followed by low surface energy POTS modification. Then, UV irradiation with a photomask resulted in the superhydrophilic− superhydrophobic micropatterns (Figure 23d). The resultant pattern can be used as a template to further fabricate site-selective functional patterning with biomedical applications.351 Patterned TiO2−SiO2 surfaces were fabricated by Ralston et al. using photolithography and plasma-enhanced CVD.352 Owing to the different wetting behavior of TiO2 and SiO2, the patterned inorganic surface showed a wettability contrast of about 40°. After the FAS modification followed by UV irradiation, a superhydrophilic−hydrophobic pattern was formed on the inorganic surfaces owing to the decomposition of FAS only on the TiO2 patches. The wettability contrast on the smooth TiO2− SiO2 surfaces was maximized to 120°. Furthermore, the resultant inorganic patterned surfaces possess excellent high-temperature resistance. Lindén et al. also fabricated hydrophilic−hydrophobic TiO2− SiO2 nanopatterns arising from the different photocatalytic properties of titania and silica.353 In order to enhance the stability of patterned wettability, a simple method was developed by Zhang et al. to fabricate TiO2−SiO2 superhydrophilic−superhydrophobic patterns.354 First, superhydrophobic SiO2 cone arrays were prepared using reactive ion etching followed by the 10063


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approach is time-saving, resulting in the superhydrophilic− superhydrophobic TiO2 pattern without a photomask. A similar strategy has also been applied for the construction of TiO2-based superhydrophilic−superhydrophobic patterns on titanium substrates through a combination of an inkjet technique and siteselective decomposition of ODP using TiO2 under UV irradiation.356 All these TiO2-based superwetting patterns can be used as the offset printing plates and were fully renewable by photocatalytic decomposition of TiO2 under UV irradiation. In the conventional systems, the superhydrophilicity on the TiO2 surface will be lost after prolonged storage in the dark. Furthermore, hydrophobic compounds should be decomposed owing to the TiO2 photocatalysis under UV irradiation, resulting in the formation of bare TiO2 surface. In order to overcome these shortcomings, a renewable superhydrophobic-superhydrophilic pattern has been fabricated using the ODS-covered superhydrophobic Al2O3 overlayers on a superhydrophilic TiO2 surface by self-assembly of boehmite particles (Figure 25).357

PFTS modification. Photoresist was patterned on the superhydrophobic SiO2 surface through the combination of spincoating and photolithography. The O2 plasma etching approach was used to remove the exposed PFTS molecules. TiO2 nanorod clusters were deposited on the above substrate using the hydrothermal process. After removal of the residual photoresist, superhydrophilic−superhydrophobic patterns with TiO2 nanorod clusters surrounded by SiO2 cone arrays were fabricated. The pattern can be simply adjusted during the photolithography process, ranging from separated circles and triangles to continuous strips and networks (Figure 24). Functional

Figure 24. Fluorescent photographs of the obtained superhydrophilic TiO2 nanorod clusters with various patterns on the surface of superhydrophobic SiO2 cone arrays, cycles (A), triangles (B), strips (C), and networks (D), where the substrate are stained with rhodamine B (A and D) and fluorescent isothiocyanate (B and C), respectively. Scale bars, 50 μm.354 Reprinted with permission from ref 354. Copyright 2010 Royal Society of Chemistry.

Figure 25. Renewable superhydrophilic−superhydrophobic pattern. (Left) Optical micrograph of the Al2O3 stripes patterned on the TiO2 thin film with a photomask of alternating 50 μm opaque and transparent areas. (Right) AFM image of the patterned Al2O3 stripe.357 Reprinted with permission from ref 357. Copyright 2009 American Chemical Society.

molecules were well preserved on the patterned substrates even after rinsing with a water stream repeatedly, showing the good stability and potential as function-integrated microchips. It was also found the patterned TiO2−SiO2 surface demonstrated photoinduced cleaning by repeatedly adsorbing and decomposing Rhodamine B for at least six cycles. During the renewal process, the superhydrophilic−superhydrophobic pattern exhibited considerable stability, where PFTS molecules were not degraded and the high wettability contrast was preserved. These stable wetting patterns should have promising applications as reusable lab-on-a-chip devices.

The Al2O3 layer possesses dual roles as a superhydrophobic layer and as a UV-blocking layer for the underlying TiO2. The surface wetting pattern can be repeatedly recovered by simple UV irradiation even after prolonged dark storage, demonstrating the practical applications in fluid microchips and microreactor arrays. 5.3. Rewritable Wetting Patterns

Through improving the above-mentioned synthesis approach, rewritable superhydrophilic−superhydrophobic patterns were prepared by Fujishima et al. on a TiO2 surface by combining the inkjet method and the site-selective decomposition of ODS under UV irradiation (Figure 26).358 Calcination of titanium foil at high temperature in air resulted in the increase of surface roughness and then the surface wettability after modification with ODS. The resultant superwetting patterns were rewritable through the removal of patterns using the photocatalytic decomposition under UV irradiation and subsequent resurfacing. Now, it is still a challenge to fabricate superhydrophilic− superhydrophobic patterns without the use of organic chemicals. Recently, TiO2−PDMS composite films have been fabricated by a sol−gel method followed by curing with UV light.359,360 A superhydrophilic−hydrophobic wettability pattern was prepared on the TiO2−PDMS composite film by selective UV irradiation with a photomask. The resultant pattern exhibited rewritable

5.2. Inkjet-Induced Wetting Patterns

The above-discussed synthesis strategies for superhydrophilic− superhydrophobic TiO2 patterns were usually time-consuming and need photomasks, which may be unsuitable for their practical applications. An inkjet technique was proposed by Fujishima et al. to fabricate TiO2-based superhydrophilic−superhydrophobic patterns on an anodized Al plate without the use of a photomask.355 This approach is composed of the following five steps: (a) preparation of TiO2 films on rough Al substrates, (2) surface modification with ODP, (3) construction of aqueous UV light-resistant ink patterns using an inkjet technique, (4) formation of the superhydrophilic state through the photocatalytic decomposition of ODP under UV irradiation, and (5) removal of the patterned aqueous ink by water washing. This 10064


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Figure 27. SEM images of TiO2 nanostructures (a) and hierarchical structures (b) with growth times of 4 h. (c) Glycerol on the Si substrate with TiO2 hierarchical structures.361 Reprinted with permission from ref 361. Copyright 2013 Royal Society of Chemistry. Figure 26. Schematic representation of the wettability pattern fabrication procedure.358 Reprinted with permission from ref 358. Copyright 2010 American Chemical Society.

superoleophobicity on the glass substrate.362 Fluorinated TiO2 nanoparticles showed super-repellency toward water, glycerol, rapeseed oil, toluene, hexadecane, silicone oil, chloroform, and tetrachloromethane. This can be ascribed to the cooperation of low surface energy PFOA, rough surface structures, and the adhesion of TiO2 particles at the bottom of liquid droplets. Recently, Meroni et al. reported amphiphobic TiO2 films possessing multiscale roughness after the siloxanes modification.363 The photocatalytic lithography approach was exploited to construct hydrophobic/oleophobic and superhydrophobicsuperhydrophilic patterns. These patterned surfaces have promising applications in a site selective adsorption of a dye molecule. Topalian et al. investigated the photoinduced SO2 fixation on anatase TiO2 films under UV irradiation in gas containing SO2 and O2 at different temperatures.364,365 It was demonstrated that, owing to the band gap excitation of TiO2, adsorbed SO2 was photo-oxidized to sulfide and sulfate surface species, resulting in the acidification of the TiO2 surface. TiO2 thin films after the photofixation of SO2 exhibited weak adhesion toward stearic acid, showing oleophobicity. This approach provided a new avenue for the construction of oleophobic surfaces through the surface acidification of TiO2. Electrospinning is an effective method to construct rough surfaces with controlled size, shape, and porosity, which is important for the final superamphiphobicity. Recently, Nair et al. fabricated superamphiphobic TiO2 nanostructures with riceshape using the electrospinning technique followed by fluorinated silane modification on glass substrates (Figure 28).366 The resultant porous TiO2 films with self-cleaning exhibited superrepellency with a low contact angle hysteresis toward water, ethylene glycol, and hexadecane. Moreover, these superamphiphobic coatings possessed exceptional mechanical and thermal stability with strong adherence to the glass substrate, demonstrating the potential for practical applications. In order to improve the repellency property of cellulose-based materials against oil, re-entrant or overhang structures should be introduced to enhance the surface roughness. The chemical etching approach is an effective method to impart surfaces with hierarchical structures. Huang et al. fabricated amphiphobic cellulose-based materials by chemical etching and successive POTS modification.367 First, a natural cellulose substance (filter paper) was etched with an alkaline solution to enhance the surface roughness. Then, ultrathin TiO2 films were deposited onto the etched filter paper by a sol−gel method. Finally, low surface energy POTS was further self-assembled onto the above

wettability without the need for organic chemicals, which is stimulated by the UV irradiation and thermal treatment.359 These TiO2−PDMS composite films with patterned wettability could be used as the offset printing plate.

6. SUPEROLEOPHOBIC TITANIUM DIOXIDE MATERIALS Superhydrophobic surfaces are ubiquitous in nature. Inspired by biomaterials with special wettability, a wide variety of artificial superhydrophobic surfaces have been prepared in recent years. However, superoleophobic surfaces are extremely rare in nature. Usually, oils and other organic liquids have a lower surface tension in comparison with water. Therefore, it is much more difficult to construct oleophobic materials, especially superoleophobic materials than superhydrophobic ones. However, a great deal of work has been devoted to the design and fabrication of oleophobic even superoleophobic materials for both fundamental research and practical applications in many different fields.70 The wettability of a solid surface is usually governed by surface geometrical structures and surface free energy. Therefore, in order to construct superoleophobic surfaces, it is essential to rationally design the surface geometry as well as surface chemical compositions. In this section, we will mainly discuss the recent developments in the design and fabrication of superoleophobic TiO2 materials in air or underwater. 6.1. Superoleophobicity in Air

Inspired by the superhydrophobic materials with micronanoscale structures in nature, Lee et al. fabricated TiO2 hierarchical structures in combination with sol−gel-based nanoimprint lithography and hydrothermal growth approaches (Figure 27).361 After modification with low surface energy HDTS, the resultant TiO2 hierarchical structures with the inherent overhang geometry exhibited self-cleaning and superamphiphobicity (both superhydrophobicity and superoleophobicity) with a low contact angle hysteresis toward water, glycerol, and diiodomethane. Furthermore, TiO2 hierarchical structures showed more stable and robust superhydrophobicity than nanostructures for water evaporation, which can be attributed to hierarchical air pockets that minimized the contact area between liquids and solids. Zhang et al. prepared PFOA modified TiO2 nanoparticles with 10065


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Figure 28. Scheme for the fabrication process of rice-shaped TiO2 nanostructures by electrospinning for creating a robust superamphiphobic coating on glass substrates.366 Reprinted with permission from ref 366. Copyright 2013 American Chemical Society.

hydrophobicity to superamphiphobicity through adjusting the electrochemical parameters, such as electrochemical etching time, electrolyte temperature, current density, and electrolyte concentration. Combining electrochemical etching and hydrothermal synthesis processes, superamphiphobic TiO2 multiwalled nanotube arrays with 8 nm diameter were fabricated by Jin et al. on a microtextured Ti foil.371 After modification with PFTS, TiO2 nanotube arrays exhibited superrepellency toward water and glycerol droplets with contact angles of 178° and 174°, respectively. The nanometer-scaled structures introduced by hydrothermally grown TiO2 nanotubes offered an effective air trapping nanostructure in enhancing the surface superamphiphobicity. Navarrini et al. fabricated amphiphobic Ti plates by anodization and postmodification with fluorinated polymers. The resultant surfaces exhibited UV-resistant properties and possessed a good adhesion between the fluorinated polymers and the anodized titanium substrate.372 Recently, Lim et al. fabricated superamphiphobic TiO2 nanotube arrays on a microstructured Ti foil using a two-step electrochemical anodization method followed by the PFTS modification.373 The resultant superamphiphobic surface exhibited good durability and stability even after long-term storage. It was found that the superamphiphobicity of TiO2 nanotube arrays was dependent on the nanotube surface morphology, which can be controlled by tuning the applied voltage and anodization time during electrochemical anodization. Using a modified liquid phase deposition process followed by the PFDTS modification, Cui et al. fabricated superamphiphobic TiO2/SWNT composite coatings on the silicon wafer substrates.374 The overlapped TiO2 clusters on the coating surface possessed overhanging structures (Figure 29). After the PFDTS modification, the wettability of composite coatings was changed from superhydrophilicity to superoleophobicity with a small sliding angle. The incorporation of SWNT increased the solid−liquid interfaces in the reaction solution owing to its large specific surface area, which facilitated the nucleation of TiO2 crystals, decreased the concentration of titanium fluoride complexes, and then improved the oil repellency.

substrate. Utilizing the above approach, the pristine hydrophilic filter paper was converted into an amphiphobic surface with both superhydrophobicity and high oleophobicity toward water and hexadecane, respectively. The layer-by-layer approach is an effective method to modify nanoparticles by electrostatic interactions, which has the potential to fabricate functional surfaces in aqueous solution and to control the surface wettability. Superamphiphobic TiO2−polymer composites were prepared by Shiratori et al. using the layer-by-layer approach involving a water-soluble copolymer.368 The resultant film exhibited repellency toward water, rapeseed oil, ethylene glycol, and hexadecane with a small sliding angle. During the layer-by-layer process, PAA and PMC were used as hydrophilic anionic polymer and hydrophobic cationic copolymer, respectively. The binding force of the strong polycation resulted in the formation of nanoparticles aggregations with broccoli type structures, which is important for the surface superamphiphobicity. Metal and its alloys are very important and irreplaceable engineered materials in our society.63 Apart from their unique properties, metal and its alloys present some property limitations, such as corrosion and biofouling by oil/water pollution. The construction of superwettability on metallic substrates is a scientifically and technologically challenging target. Recently, in order to extend their applications, a variety of superamphiphobic metallic surfaces have been fabricated by using different processes.63 Utilizing anodization and laser micromachining, Zhou et al. reported engineered superamphiphobic TiO2 on Ti plates.369 In order to increase the surface roughness, Ti plates were first anodized in a salt solution to produce microstructured surfaces. Alternatively, the laser micromachining technology was used to produce patterned Ti substrates with a controllable manner. Further anodization was applied to generate thin, wellaligned TiO2 nanotube films with patterned lines, salients, and pits. After modification with PFTS, the resultant TiO2 surfaces showed superamphiphobicity toward a broad range of liquids including water, hexadecane, glycerol, and CH2I2. Utilizing the electrochemical etching approach followed by FAS modification, Xu et al. reported the preparation of superamphiphobic Ti surfaces containing TiO2 with re-entrant geometries.370 The resultant surfaces showed super-repellency toward water, glycerol, and hexadecane with a high static contact angle of above 150° and a low sliding angle of about 1−2°. The re-entrant geometry is the key to generate superoleophobicity. The surface wettability of Ti substrates could be tailored from

6.2. Underwater Superoleophobicity

In the preceding sections, we have discussed the progress in TiO2 with special wettability in the air environment. Recently, underwater superoleophobic materials have attracted much attention arising from their important applications in marine antifouling, oil spill cleanup, etc.70 In nature, fishes rather than seabirds are known to be well protected from contamination by 10066


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water. This can be attributed to the formation of the thin water film consisting of adsorbed water molecules, which prevented oil droplets from contacting the TiO2 surface (Figure 30).

Figure 30. Images of oil wettability conversion of TiO2 surfaces in water by UV irradiation.388 Reprinted with permission from ref 388. Copyright 2013 American Chemical Society. Figure 29. Superoleophobicity realized on a TiO2/SWNT composite coating surface. (a) SEM images of the TiO2/SWNT porous coating on a silicon wafer synthesized using 2 μg/mL SWNT. (b) The cross-section view of the coating shows overhanging structures formed by the TiO2/ SWNT clusters. (c) The high-magnification view of the coating surface shows that the surface of TiO2 particles is covered by nanocrystals, forming hierarchical surface structures. (d) A droplet of silicone oil on top of the above coating. The inset shows a droplet of silicone oil rolling down a tilted coating surface.374 Reprinted with permission from ref 374. Copyright 2011 American Chemical Society.

Furthermore, the surface self-cleaning effect induced by the photocatalytic decomposition of surface organic contaminant also contributed to the high underwater superoleophobicity. Although the resultant superhydrophilic and underwater superoleophobic TiO2 surfaces lost their superwettability after the contamination treatment, their original wetting state can be recovered by UV irradiation without damage after several switching cycles, exhibited reversibly photoswitchable wetting properties.

oil pollution, exhibiting underwater self-cleaning and antifouling properties.70,375−377 For example, grass carp scales (Ctenopharyngodon idella) present superoleophilicity in air and superoleophobicity in water due to their special surface multiscale structures and chemical compositions.375 A thin water film was expected to be present between the fish scales and the oil droplets underwater, resulting in underwater superoleophobicity in the oil−water−solid system. Usually, an oleophilic surface in air (oilair−solid system) will convert oleophobic underwater (oil− water−solid system). The contact angle with water, oil in air and oil in water can be expressed by the following Young equation: cos θ W = (γSA − γSW )/γWA , cos θOW = (γSW − γSO)/γOW

7. TITANIUM DIOXIDE SURFACES WITH SWITCHABLE WETTABILITY Smart surfaces with tunable wettability that can reversibly switch between hydrophobicity (superhydrophobicity) and hydrophilicity (superhydrophilicity) have aroused much interest owing to their promising applications from both a scientific and an industrial standpoint.54,64,389,390 TiO2 is an important photoresponsive material. Since the discovery of photoinduced wettability switching of TiO2 by Fujishima et al.,4 much attention has been focused on the manipulation of surface wettability. Inspired by the photosensitive TiO2, many different synthesis strategies have been developed to construct TiO2-based smart materials with tunable wettability between hydrophobicity (superhydrophobicity) and hydrophilicity (superhydrophilicity).94,391−395 In this section, we will mainly discuss the recent advances in the switchable wettability of TiO2 surfaces under the external stimulation, such as light, temperature, electrical potentials, laser, and multistimuli.

cos θO = (γSA − γSO)/γOA , (7)

where γ is the interface free energy and A, O, S, and W refer to air, oil, solid, and water, respectively. For a hydrophilic surface (γSW < γSA) and an oleophilic surface in air (γSO < γSA), it can be switched into the underwater oleophobic surface (γSO > γSW).378 Therefore, oil contaminants on such surfaces can be washed away when immersed in water, exhibiting underwater selfcleaning. Inspired by underwater superoleophobic surfaces found in nature, a wide variety of underwater superoleophobic materials have been fabricated by using different synthesis strategies.41,59,379−386 These research works provide an avenue for the underwater applications in self-cleaning, microfluidics, biotechnologies, oil/water separation, antifouling, etc. The research on underwater superoleophobicity is in its infancy, but it is a rapidly growing and promising field.387 Recently, Miyake et al. reported the fabrication of photoinduced underwater superoleophobicity on a flat TiO2 surface prepared by a sol−gel method or by calcination of a Ti plate.388 The resultant amphiphilic TiO2 surface produced by UV irradiation showed underwater superoleophobicity and low oil adhesion in

7.1. UV Light

UV light is one of the most important external stimuli. A great number of TiO2−based materials with UV-driven switchable wettability have been fabricated through the alternation of UV irradiation and dark storage.4,54,64,94,389−396 Hashimoto et al. reported the photoinduced wettability conversion of TiO2/WO3 layered films.397 Although the top surface was fully covered by TiO2, the existence of WO3 enhanced the wettability conversion rate using a 10-W fluorescent light bulb with a 10 μW/cm2 UV intensity. This can be attributed to the following considerations, (i) the presence of charge transfer between TiO2 and WO3 layers. Photogenerated holes accumulate in the TiO2 layer, whereas electrons accumulate in the WO3 layer; (ii) electron−hole pairs produced by visible light. These photogenerated holes in the WO3 layer can initiate the surface reactions of TiO2. Using other synthetic strategies, they also fabricated a number of TiO2−based 10067


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Figure 31. (a) Low-magnification SEM image of a TiO2 nanorod film deposited on a glass wafer. (b) SEM image of a single papilla at high magnification. (c) Photographs of a spherical water droplet with a CA of (154 ± 1.3)° (left) and a flat water film with a CA of 0° (right) before and after the films were exposed to UV illumination, respectively. (d) Reversible superhydrophobicity/superhydrophilicity transition of the as-prepared films by alternating UV irradiation and storage in the dark.94 Reprinted with permission from ref 94. Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

the dark, respectively. Under UV irradiation, TiO2-coated aerogels can absorb water 16 times their own weight, showing the superabsorbent property. After the dark storage for 2 weeks, the original absorption and wetting properties can be recovered. TiO2-coated nanocellulose aerogels with high porosity also possessed photocatalytic activity, demonstrating the potential applications in water purification. Since the discovery of photoinduced superhydrophilicity on a hydrophilic TiO2 surface, a number of efforts were devoted to amplifying the photochemically driven response of TiO2 surface from superhydrophobicity to superhydrophilicity. Jiang et al. first reported the fabrication of TiO2 nanorod films possessing reversible switching between superhydrophobicity and superhydrophilicity (Figure 31).94 TiO 2 nanorod films with hierarchical structures were fabricated on glass substrates using a low-temperature hydrothermal approach. The water contact angle of rough TiO2 nanorod films is about 154°, showing superhydrophobicity. After UV light irradiation, surface wettability of TiO2 nanorods transformed from superhydrophobicity to superhydrophilicity. After the storage in the dark for 2 weeks, the original superhydrophobicity was restored. This process can be repeated for several cycles, demonstrating a good reversibility. The cooperation of the special surface hierarchical structures, the surface photosensitivity, and the orientation of crystal planes resulted in the switchable superwettability. Recently, Stepien et al. fabricated a superhydrophobic TiO2 surface using a liquid flame spray method on paper or paperboard substrates.404 They also investigated the connection between surface wettability and surface chemistry on the nanoscale during UVA irradiation and heat treatment using the time-of-flight secondary ion mass spectrometry. The initial superhydrophobic surface can be converted into hydrophilicity after UVA irradiation. Heat treatment converted the hydrophilic surface back into a hydrophobic surface, arising from the increase in the number of carboxyl-terminated molecules. Chen et al. also reported the reversible superhydrophobic-superhydrophilic transition of TiO2 nanostrawberry films by alternating UV illumination and dark storage.394 The aligned TiO2 nanostrawberry film was prepared using a simple seeded growth method at a low temperature. The as-prepared film showed superhydrophobicity. Upon UV irradiation, water droplets spread out thoroughly on the surface immediately, exhibiting superhydrophilicity. After the dark storage for several weeks, superhydrophobicity of the film was restored. Huang et al. fabricated superhydrophobic cellulose material through assembly of a photosensitive azobenzene derivative monolayer on a TiO2

materials with enhanced photoinduced surface wettability conversion on TiO2 films by alternate UV illumination and the dark storage.84,86,87,266,398 Koumoto et al. fabricated amorphous TiO2 thin films on self-assembled monolayers through the peroxotitanate-complex deposition and liquid-phase deposition.201 After UV irradiation for 1 min, amorphous TiO2 thin films exhibited superhydrophilicity. The surface wettability of amorphous TiO2 films can be switched between superhydrophilicity and hydrophobicity by alternate UV light irradiation and storage in an atmosphere filled with organic gases. The irradiation-induced wettability conversion can be ascribed to the transformation of Ti−OH to the Ti−O−Ti groups. Bahnemann et al. prepared transparent hydrophilic TiO2/SiO2 thin films on polycarbonate through a dip-coating process.399 The addition of SiO2 resulted in an improvement of the optical properties. After UV irradiation, the wettability of TiO2/SiO2 films was changed from hydrophilicity to superhydrophilicity. The water contact angle will increased after the dark storage. Electrospinning proved to be a simple approach for the large-scale fabrication of polymer nanofibers. In order to fabricate a functional mat with tunable wettability, Yoon et al. prepared a three-dimensional nanocomposite mat using electrospun polystyrene nanofibers and electrosprayed TiO2 nanoparticles.400 Under UV light irradiation, the resultant nanocomposite mat showed efficient switching between superhydrophobicity to hydrophilicity arising from the hydrophobic polystyrene and hydrophilic TiO2 with photocatalytic properties. Mimicking natural mineralization, Kim et al. synthesized Ndoped CNTs/TiO2 core/shell nanowires.401 These core/shell nanowires exhibited enhanced visible light photocatalysis. Furthermore, Stimuli-responsive wettability driven by UV light or electric potential was also observed on the core/shell nanowire surface. Athanassiou et al. reported the reversibly light-switchable wettability of SU-8/TiO2 surfaces with multiscale roughness.402 The hybrid organic/inorganic surfaces consist of photolithographically patterned SU-8 micropillars covered by TiO2 nanorods. The combination of surface structures and chemical compositions resulted in UV-driven reversible transitions between a highly hydrophobic state and a highly hydrophilic state. The native hydrophobicity can be restored after a dark storage of a few weeks. Inspired by the highly hydrophilic character of nanocellulose hydrogels and aerogels, Ikkala et al. reported the chemical vapor deposition of a TiO2 film on lightweight native nanocellulose aerogels.403 TiO2-coated aerogels exhibited photoswitching between water-superabsorbent and water-repellent states upon UV irradiation and storage in 10068


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film precoated filter paper surfaces.405 The resultant cellulose sheet exhibited reversible wettability between superhydrophobicity and superhydrophilicity through alternating UV irradiation and dark storage processes. The conformational transformation of the photosensitive azobenzene derivative and the surface changes of the TiO2 gel layer during UV irradiation and dark storage are responsible for the photoinduced reversible superwettability. Borras et al. reported the reversible superhydrophobic to superhydrophilic conversion on Ag@TiO2 composite nanofiber surfaces.406 Core−shell Ag@TiO2 nanofibers were fabricated at a low temperature by plasma-enhanced chemical vapor deposition. Under UV irradiation, the nanofiber surface was converted into a superhydrophilic state. When the samples were heated or illuminated with visible light, the original superhydrophobic state can be restored. Wang et al. prepared polystyrene/TiO2 nanocomposite films on glass substrates by a casting method.407 These films exhibited UV-driven reversible switching between superhydrophobicity and superhydrophilicity. The superhydrophobic composite surfaces can be converted into superhydrophilic ones by low-intensity UV irradiation, which can be recovered after the heat treatment.

photoinduced superhydrophilicity and thermally stimulated switchable wettability on TiO2−PDMS surfaces provided an avenue for the fabrication of rewritable patterns controlled by external stimuli. Hashimoto et al. prepared anatase TiO2 thin films on SrTiO3 substrates through a metalorganic chemical vapor deposition method.413 It was found that these TiO2 films possessed reversible wetting properties between hydrophilicity and hydrophobicity under the alternation of UV and visible light containing infrared ray irradiation. The UV-induced hydrophobic−hydrophilic conversion can be ascribed to the increase in dissociated water adsorption on the film surface, while the elimination of surface hydroxyl by the thermal process resulted in the visible light-induced hydrophilic−hydrophobic conversion without an interband transition. Recently, without surface chemical modification, Lyons et al. fabricated superhydrophobic TiO2−HDPE nanocomposite films with self-cleaning using a template lamination method.314 The TiO2−HDPE nanocomposite surface presented a UV-thermal induced reversible wettability between superhydrophobicity and hydrophilicity, which can be repeated over numerous cycles. The wetting conversion from superhydrophobicity to hydrophilicity can be attributed to the hydrolysis of the TiO2 nanoparticle surface upon UV irradiation, which can be accelerated when the surface was immersed in water. The heat treatment resulted in the decrease of Ti−O−H bonds and the restoration of hydrophobic Ti−O bonds, showing the primary superhydrophobic state. (Figure 32)

7.2. Visible Light

Recent studies demonstrated that N-doping is a promising method adopted for making TiO2 visible-light-absorbing. The wetting−dewetting properties of N-doped TiO2 surfaces have been studied in recent years.206,408,409 Burda et al. fabricated Ndoped anatase TiO2 nanorods using a hydrothermal method.410 These nanorods exhibited visible-light-driven reversible and switchable wettability between hydrophobicity and hydrophilicity. Under visible light irradiation, hydrophobic N-doped TiO2 surfaces was transformed into hydrophilic ones, which can be reversible to hydrophobic surfaces after the dark storage for 2 weeks or the heat at 120 °C. It was found that the photocatalytic activity of N-doped TiO2 is dependent on the surface wettability. Mathews et al. investigated the physicochemical processes and kinetics of sunlight-induced reversible and switchable hydrophobic to superhydrophilic transition on transparent N-doped TiO2 films.411 The film was prepared using ultrasonic spray pyrolysis. The kinetic rates of the conversion from hydrophobicity to superhydrophilicity under sunlight and the transition from superhydrophilicity to hydrophobicity under dark were about 0.215 min−1 and 2.03 × 10−4 min−1, respectively. In addition to the above-mentioned external stimuli, nanosecond excimer laser irradiation can be used to achieve rapid wetting switching of TiO2-based single crystalline heterostructures from a hydrophobic to a hydrophilic state.412 The formation of oxygen vacancies on the surface should be responsible for the observed superhydrophilic behavior. The laser-induced enhancement of hydrophilicity can be reversed by exposing the samples at ambient atmosphere to remove surface oxygen vacancies through the oxygen absorption.

7.4. Electric Potential

One way to reversibly switch the surface wettability is to apply an electric field between the surface and a liquid droplet. The

7.3. Thermal Stimulus

Fujishima et al. fabricated TiO2−PDMS composite films by a sol−gel method.359 After the hot water treatment, the film was transparent to visible light and exhibited photoinduced superhydrophilicity under UV irradiation owing to the presence of anatase TiO2. These TiO2−PDMS composite films could recover their initial wettability and the recovery rate of the water contact angles significantly increased at high temperature. This can be attributed to the conformational changes in the molecular bonding around Ti atoms and replacement of the adsorbed hydroxyl groups by oxygen. The combination of

Figure 32. (a and b) Reversible wettability changes during cyclic alternation of UV Illumination for 30 min with water, and heating at 105 °C for 1.5 h. (c) The reversible wetting-non wetting transmission on the fabricated TiO2-polymer nanocomposite surface with hierarchical structures.314 Reprinted with permission from ref 314. Copyright 2013 American Chemical Society. 10069


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formation of hydrophilic COO− groups, exhibiting superhydrophilicity. When the NaOH-treated films were immersed in the hydrochloric acid solution, COO− groups reverted to COOH groups. These uncharged COOH groups produced compact acrylic acid chains. The intramolecular hydrogen bonds between COOH groups became dominant. Finally, surfaces were mainly composed of hydrophobic macromolecular chains, showing superhydrophobicity.

advantage of using electrical potential as an external stimulus is its ability to control the surface chemistry and/or morphology. For example, when an external electric potential is applied between a droplet and a hydrophobic surface, the surface energy is lowered arising from the net electric charge appearing at the interface, resulting in hydrophilicity. Papadopoulou et al. reported the electrowetting properties of TiO2 nanostructured thin films.414 The TiO2 thin film was fabricated on crystalline Si substrates using pulsed laser deposition and subsequently coated by a thin dielectric layer. Reversible electrowetting at low voltages was observed when nanostructured TiO2 thin films were used as conductive electrodes. Upon the application of the external electric field between a water droplet and the TiO2 film, the water contact angle was decreased. After the removal of the applied voltage, the water droplet returned to its initial hydrophobic state. Considering the photoswitchable property of TiO2, this film could be used to design dual-functional coatings possessing both light- and electric field-induced wettability switching.

7.6. Switchable Oil Wettability

Current research on smart surfaces with switchable wettability was mainly focused on the wetting transition between superhydrophobicity and superhydrophilicity under external stimuli, such as light, electrical potential, magnetic field, temperature, pH, enthalpy, selected solvent, and others.54,64,390,416 Recently, materials surfaces with switchable oil wettability and adhesion have attracted a great deal of attention, arising from their considerable importance in both fundamental research and practical applications. TiO2 was demonstrated to be an effective photoresponsive material for the realization of switchable oleophobicity under the UV light stimulus. Hierarchical TiO2 surfaces were fabricated by Zhou et al. on engineered Ti substrates by the combination of anodization and laser micromachining.369 After postmodification with PFTS, the resultant TiO2/Ti surfaces exhibited superamphiphobicity toward water, glycerol, hexadecane, and CH2I2. By alternating UV irradiation and heat treatment, these TiO2 surfaces demonstrated the controllable oleophobicity and the switchable oil adhesion between sliding superoleophobicity and sticky superoleophobicity. Under UV illumination, the superoleophobic surface was converted to superoleophilic one, which can be attributed to the generation of surface hydroxyl groups and the fluorine content decrease. The superoleophilic surface can revert to superoleophobic one after the dark storage for 2 weeks, owing to the rearrangement of the fluoride alkyl chain. Furthermore, after exposing to the UV light with a patterned mask, the adhesion property of the superoleophobic TiO2 film was changed from a low adhesion to a high adhesion. The recovery time from sticky to sliding superhydrophobicity was dependent on the heating temperature. Lim et al. reported superamphiphobic TiO2 nanotube arrays on a microstructured Ti substrate by the combination a two-step electrochemical anodization and postmodification with PFTS.373 The resultant TiO2 arrays exhibited switchable wettability for both water and oil from superamphiphobicity to superamphiphilicity through the alternation of air plasma treatment and surface fluorination. Combining the composite liquid phase deposition and the PFDTS postmodification, Cui et al. fabricated superoleophobic TiO2/SWNT coatings with overhang structures.374 The wettability of the coatings can be adjusted from superamphiphobicity to superamphiphilicity toward different liquids with surface tension ranging from 27 mN/m (hexadecane) to 72 mN/m (water). By regulating the UV illumination dose, liquids with surface tension difference smaller than 5 mN/m can exist in converse wetting states on the same surface. This can be attributed to the gradual photocatalytic decomposition of PFDTS on the surface under UV irradition.

7.5. Multi-Stimuli

As above-discussed reversible wettability, a variety of stimuliresponsive TiO2 surfaces have been fabricated. However, these TiO2 materials are only responsive to one kind of external stimuli. For some applications, multiple-responsive materials are required to control surface wettability. Recently, Wu et al. fabricated superhydrophobic TiO2−P(S-BA-AA) films with multiresponsive and reversibly tunable wettability (Figure 33).415 The hybrid film was fabricated through casting the

Figure 33. Reversible superhydrophobicity−superhydrophilicity transition of the as-obtained nanocomposite films (a) under UV irradiation and 150 °C treatment and (b) under treatment with pH 12 NaOH and pH 2 HCl solutions. (c) Compact conformation of PAA chains and intramolecular hydrogen bonding at pH 2, and the loosely coiled conformation of PAA chains and intermolecular hydrogen bonding at pH 12.415 Reprinted with permission from ref 415. Copyright 2013 Royal Society of Chemistry.

mixture of TiO2 nanoparticles and P(S-BA-AA) solution in tetrahydrofuran on substrates. The resultant film exhibited reversibly tunable wettability between superhydrophilicity and superhydrophobicity under external stimuli (UV light/heat, or acid/base solutions). The switchable wettability can be ascribed to changes in the surface compositions caused by the external stimulation. For example, COOH groups can be used to construct pH-responsive materials. The treatment with the NaOH solution resulted in the deprotonation of COOH and the

8. APPLICATION OF TITANIUM DIOXIDE MATERIALS WITH SPECIAL WETTABILITY Arising from its stable, nontoxic, cheap, and special physicochemical characteristics, TiO2 is one of the most basic materials in our daily life, which has a wide variety of practical 10070


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applications in different fields.5,19,22,212,417,418 This section will focus on the recent advances in the applications of TiO2 materials with special wettability in antibacteria, anticorrosion, antifogging, biomedical, device, fluid transportation, liquids separation, offset printing and liquid reprography, self-cleaning, site-selective functional patterning, water condensation, as well as agricultural and environmental fields.

cellulose surfaces owing to the fewer location sites.272 Although amphiphobic TiO2-coated cellulose materials possess promising antibacteria properties, fluorinated POTS compounds are introduced to reduce the surface energy, which are difficult to decompose. Therefore, the security to human health should be considered during the practical applications. Parkin et al. prepared hydrophilic TiO2 and Ag-doped TiO2 coatings on glass microscope slides using a sol−gel dip-coating method.423 It was found that Ag-doped TiO2 coatings showed more photocatalytically and antimicrobially active than pure TiO2 coatings under UV light irradiation. However, in the dark, no antimicrobial activity was observed. In order to achieve the antibacterial property under white light, Parkin et al. also fabricated N-doped TiO2 with visible light induced superhydrophilicity.207 The photocatalytic activity of N-TiO2 can be further enhanced by incorporating Ag nanoparticle islands. Representative antibacterial properties were investigated using E. coli as a Gram-negative organism and EMRSA-16 as a Grampositive organism. It was found that these Ag−N−TiO2 films can be used as antimicrobial surfaces to inhibit the development of bacterial colonies that act as a reservoir for infection. The primary antibacterial effect was ascribed to the silver release. Moreover, the antimicrobial properties of Ag−N−TiO2 can be improved when exposed to white light as no living E. coli cells were detectable and a reduction of about 2 log10 for MRSA was achieved, compared to uncoated samples. They also prepared sulfur-doped TiO2 thin films by atmospheric pressure chemical vapor deposition.424 These sulfur-doped TiO2 films presented superior white light induced photocatalysis, photoinduced superhydrophilicity, and self-cleaning compared to the industrial products Activ and BIOCLEAN. This can be attributed to the incorporation of sulfur atoms in the lattice and the formation of radical species on the surface. These visible light photocatalysts have real potential as agents against both bacteria and stearic acid commonly found in UK hospitals.425 Titanium and its alloys have been widely used as medical implants arising from their reliable biocompatibility and mechanical properties.426 However, the inhibition of bacterial colonization and formation on implant surfaces is still a challenge. Owing to the biocompatible and antibacterial properties, TiO2 has exhibited great promise as a preferred platform to achieve the bactericidal effect on biomedical implant surfaces.427−432 In 2005, Rasmusson et al. reported a 10-year follow-up study of TiO2−blasted implants.427 The TiO blast implant was made from commercially pure titanium, which was blasted with TiO2 particles, exhibiting long-term stability. Recently, in order to avoid implant-associated infections, Wang et al. fabricated a multilayered coating on Ti for the controlled release of antimicrobial peptides.430 Hydrophilic films impregnated with broad-spectrum antimicrobial peptide were fabricated through a layer-by-layer assembly of TiO2 nanotubes, calcium phosphate, and phospholipid. The resultant coating exhibited high efficiency against the in vitro growth of Staphylococcus aureus and Pseudomonas aeruginosa bacteria attached on the implants and no cytotoxicity toward osteoblast-like cells. Currently, cancer treatment has emerged as one of the most serious global issue. Photodynamic therapy was regarded as a promising and alternative noninvasive approach for cancer treatment.433 Arising from its photocatalytic and biocompatible properties, TiO2 has potential applications in medicine as a new anticancer modality. In 1986, Fujishima et al. reported that TiO2 photocatalysts can be used to kill tumor cells.434 In 1992, Fujishima et al. further investigated the antitumor activity of

8.1. Antibacteria

Adhesion plays a major role in the bacterial lifestyle. Bacterial adhesion is the initial step in colonization and biofilm formation, which is detrimental to both healthcare and industrial processes.419 A number of efforts have been devoted to the development of antibacterial materials using different synthesis strategies. In recent years, the antibacteria property of TiO2based nanomaterials has been intensively investigated owing to their special photocatalytic and self-cleaning characteristics. In this section, we will mainly discuss the antibacteria property of TiO2 surfaces with special wettability. One of the most important applications of TiO2 is to kill bacteria on its surfaces. TiO2-based materials under UV or visible light irradiation produce a strong oxidative effect and can be used as a photocatalytic disinfectant without the need of electrical power or chemical reagents. This process is nonpoisonous and environmentally friendly. Fujishima et al. first reported the photocatalytic bactericidal effect of TiO2 thin films.420 E. coli suspension (150 μL, 2 × 105 cells mL−1, total 3 × 104 cells) was pipetted on a TiO2-coated glass plate. Under the irradiation with a 15 W black light for 1 h, no survival E. coli cell was found. In the absence of TiO2, UV light only caused 50% sterilization within 4 h. This indicated that the efficient bactericidal effect can be attributed to the photocatalytic reaction of the TiO2 film, which can be observed even when the cells were separated from the TiO2 surface with a porous membrane. It was found that hydrogen peroxide was one of the active agents. In 1999, Fujishima et al. also fabricated antibacterial tiles covered with a TiO2−Cu composite coating.421 Arising from their effective bactericidal properties under UV light, the resultant tiles have been commercialized as the floor and walls of a hospital operating room. In order to achieve visible-light and even dark sensitive antibacterial effect, recently, Hashimoto et al. reported hybrid CuxO-TiO2 nanocomposites as a risk-reduction material in indoor environments through the deposition of nanocluster mixtures (CuI and CuII species) on TiO2 surfaces.422 It was found that CuII species endow TiO2 with efficient visible-light photocatalysis, whereas the CuI species impart antipathogenic properties under dark conditions. These nanocomposites also exhibited long-term stability and promising applications as air purification in private and public places. Utilizing the chemical etching approach followed by POTS modification, Huang et al. prepared amphiphobic TiO2-coated cellulose materials.367 The resultant amphiphobic composite film inhibited the adhesion of bacteria such as lysogenic E. coli. During the dilution processes by 0, 10, 100, and 1000 times, the adhesion inhibition effect of the amphiphobic sheet was consistent with no colony on the agar surface after incubation. The adhesion inhibition effect toward E. coli can be ascribed to the following considerations, (a) the high content of air trapped in the surface curvatures owing to superhydrophobicity and oleophobicity, which minimized the contact between the bacterial aqueous suspension and cellulose surfaces and (b) the nanoscale curvature, which was smaller than bacterial cell diameters in scale, reduced the adhesion strength between the microbe and 10071


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photoexcited TiO2 particles in vitro and in vivo.421,435 It was found that the presence of TiO2 (50 μg/mL) was effective to kill HeLa cells under Hg lamp irradiation for 10 min. Furthermore, photoexcited TiO2 also significantly inhibited the growth of HeLa cells implanted in nude mice. Inspired by these pioneering works, many different strategies have been developed to design TiO2-based materials with improved antitumor effect.436−440 For example, in order to increase the selective recognition of cancer cells, Rozhkova et al. fabricated TiO2 nanoparticles, which were covalently tethered to an antibody using a dihydroxybenzene bivalent linker.439 The resultant bioconjugated nanoparticles exhibited selective recognition for glioblastoma multiforme and away from normal brain cells under visible light. 8.2. Anticorrosion

Metals are irreplaceable engineered materials in daily life and industrial fields, which are the major workhorse in our society and will remain so in the future. However, the metal corrosion is ubiquitous. A wide variety of approaches have been developed to inhibit the corrosive reaction, including the use of oil paint and chromate-containing pigments as anticorrosion coatings. Concerning the human health and environmental effects, more effective solutions should be invented to solve the global corrosion issue.63 It was found that superhydrophobic surfaces can serve as an effective barrier to prevent water, moisture, corrosive ions, and atmospheric oxygen from contacting with the metal surface.63 Recently, superhydrophobic surfaces have been used to enhance the corrosion resistance of metals, which have received much attention arising from their important practical applications.63,271,441 Inspired by the natural materials with special wettability, a number of pioneering works have reported the design and construction of superhydrophobic coatings for anticorrosion on different metal substrates through the optimal combination of surface structures and surface chemical compositions.442−450 In order to evaluate the corrosion resistance of superhydrophobic surfaces, polarization curves in a Tafel model were usually conducted through the electrochemical approach.451−455 Important corrosion parameters, such as corrosion current density (ICorr) and corrosion potential (ECorr) can be calculated by the Tafel extrapolation method. It was reported that the anticorrosion efficiency of superhydrophobic surfaces could be evaluated using the following equation:456

Figure 34. Tafel plots for (a) bare cold-rolled steel, (b) electroactive epoxy-coated, and (c) superhydrophobic electroactive epoxy-coated electrodes measured in 3.5 wt % NaCl aqueous solution; (d) bare coldrolled steel, (e) electroactive epoxy-coated, and (f) superhydrophobic electroactive epoxy-coated electrodes measured after immersion in 3.5 wt % NaCl aqueous solution for 7 days. The water contact angles for pristine, electroactive epoxy-coated, and superhydrophobic electroactive epoxy-coated cold-rolled steel are about 72°, 97°, and 155°, respectively.455 Reprinted with permission from ref 455. Copyright 2011 American Chemical Society.

can be attributed to the cooperation of special surface hierarchical structures and the low surface energy POTS coating. The resultant superhydrophobic samples showed a stable corrosion resistance even after immersing in the NaCl solution for 90 days, arising from the combination of compact film structures and the superior water repellent property. Through dip-coating from the TiO2 precursor solution followed by heattreatment and FAS modification, a superhydrophobic TiO2 film with corrosion resistance toward strong acid or alkali solutions was fabricated by Pan et al. on various metal or ceramic substrates.458 Al2O3 and TiO2 atomic layer deposition were employed by George et al. to fabricate an ultrathin barrier film on copper substrates for water corrosion resistance.459 Water corrosion resistance was observed for TiO2 capping layers with nanoscale thickness on the Al2O3 adhesion layers for 80 days at 90 °C. However, Al2O3 or TiO2 atomic layer deposition alone was not effective to inhibit water corrosion resistance. Lin et al. prepared N-, S- and Cl-modified nano-TiO2 coatings on stainless steel through a sol−gel and dip-coating approach.460 It was found that the N-modified TiO2 nanocoating possessed the highest corrosion resistance among the prepared coatings. The existence of N in TiO2 coatings was beneficial to improve the compact structure and enhance the hydrophobic property.

anticorrosion efficiency(%) = 100 × (ICorr − ICorr(C))/ICorr (8)

where ICorr and ICorr(C) are the corrosion current density values of pristine and superhydrphobic surfaces, respectively. Yeh et al. investigated the effect of surface wettability on corrosion resistance, including hydrophilic bare steel, hydrophobic smooth electroactive epoxy-coated steel, and superhydrophobic electroactive epoxy-coated steel.455 It was found that higher hydrophobicity resulted in the lower corrosion current (Figure 34). Even after immersion in a saline solution for 7 days, superhydrophobic surfaces still exhibited more positive corrosion potential and lower corrosion current in comparison with both hydrophobic and hydrophilic surfaces, demonstrating the stable anticorrosion property of superhydrophobic surfaces. Here, we will only discuss the anticorrosion property of superhydrophobic TiO2-basd surfaces. Chen et al. prepared a superhydrophobic film as an effective corrosion barrier on the electrochemical oxidized titania/ titanium substrate.457 The formation of superhydrophobicity

8.3. Antifogging

Fogging is ubiquitous and it frequently occurs on the surface of cold glasses, eyeglasses, windshields, and other substrates. This nuisance can be effectively solved by controlling the interaction between liquids and surfaces. For example, superhydrophilicity can suppress the fogging behavior arising from the formation of a continuous water film on the substrate. Therefore, the scattering of the light caused by condensed water droplets can be eliminated on superhydrophilic surfaces. In 1997, the transformation of TiO2 films from hydrophobicity to superhydrophilicity was reported by Fujishima et al.4 The inducement of UV light resulted in the photoinduced antifogging and self10072


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candidates for use as photovoltaic cells. Wang et al. fabricated porous TiO2−SiO2 bilayer films through spin coating of SiO2 and TiO2 sol with poly(ethylene glycol) and subsequent annealing.465 It was found that the porous TiO2−SiO2 films possessed high transmittance and long-term superhydrophilicity without UV illumination. Owing to the superhydrophilicity and short spreading time, these films exhibited antifogging, where the light scattering caused by condensed water droplets was eliminated arising from the formation of a thin sheet-like water membrane. Utilizing the free-template approach, Huang et al. reported porous TiO2−SiO2 thin films through a sol−gel process.466 Without UV irradiation, these porous films showed superhydrophilic and antifogging properties originating from the synergistic effect of SiO2 and phase-separation-induced porous structures. Rubner et al. prepared multifunctional TiO2−SiO2 films with antifogging, superhydrophilicity, antireflection, and self-cleaning properties through a layer-by-layer deposition.467 Positively charged TiO2 nanoparticles and negatively charged SiO2 nanoparticles were used to prepare the multilayer coatings. These SiO2−TiO2 films can retain their superhydrophilicity even after dark storage for 60 days. The presence of nanopores in TiO2−SiO2 multilayer coatings resulted in nanowicking of water into the network, showing superhydrophilicity and antifogging properties (Figure 35). The superhydrophilicity of contaminated SiO2−TiO2 coatings can be readily recovered and retained after UV irradiation owing to the presence of TiO2 nanoparticles. An electrophoretic deposition method was developed by Chen et al. to fabricate transparent cross-aligned superhydrophobic TiO2-based nanobelt thin films followed FAS modification.468 By adjusting the deposition time, the adhesive property of

cleaning. This research milestone opens up a new chapter for TiO2 applications and results in a variety of exploratory frontiers ranging from antifogging to self-cleaning. Recently, a great number of synthetic strategies have been developed to fabricate antifogging surfaces through the rational design of surface wettability. Since the discovery of UV light induced superhydrophilicity,4 TiO2 has been widely investigated and used in antifogging and related fields.6 However, light-induced superhydrophilicity of TiO2 surfaces cannot maintain for a long period in the absence of UV irradiation. From a practical application of view, it is desired to design the permanent superhydrophilic TiO2 surfaces independent of UV light illumination. Therefore, in this section, the superhydrophilicity-induced antifogging with and without UV activation will be discussed separately. Generally, SiO2 coatings are superhydrophilic owing to the presence of surface hydroxyl groups, high surface roughness, and voids absorbing water. However, the superhydrophilicity of SiO2 coatings would deteriorate when organic contaminants are adsorbed on their surfaces. TiO2 coatings can show superhydrophilicity after UV irradiation, but their transmittance is usually lower than that of SiO2 coatings arising from the high reflective index of TiO2.191 In order to fabricate stable superhydrophilic surfaces with high transmittance, a classical method was developed to prepare a composite coating containing both TiO2 and SiO2. The layer-by-layer assembly method is a convenient approach for the fabrication of antireflective and self-cleaning coatings with tailored chemical composition and controllable surface structures on different substrates. Pratsinis et al. fabricated transparent nanostructured TiO2 and SiO2−TiO2 films on glass substrates using the rapid flame deposition and in situ annealing.461 Under normal solar radiation, all these films exhibited the antifogging property. Without UV radiation, pure TiO2 coatings lost their superhydrophilic and antifogging properties. However, the resultant SiO2−TiO2 coatings presented the antifogging performance under all conditions, which can be attributed to the photocatalytic effect of TiO2 and the superhydrophilicity of flamemade SiO2. Recently, He et al. fabricated a series of raspberry-like TiO2− SiO2 nanoparticles via a layer-by-layer assembly approach followed by calcination, where TiO2−SiO2 core−shell nanoparticles and SiO2 nanoparticles were used as building blocks.462 The resultant TiO2−SiO2 coatings on glass substrates exhibited superhydrophilicity, antifogging, antireflection, and self-cleaning. The maximum transmittance reached as high as 97%. The superhydrophilicity of raspberry-like TiO2−SiO2 nanoparticle coatings can be ascribed to the combined hydrophilic properties of TiO2 and SiO2. Without UV irradiation, the resultant coatings exhibited good antifogging property, arising from the rough surface with high porosity and abundant Si−OH groups. They also fabricated porous coatings consisting of nanoflakes on glass substrates through the one-step hydrothermal alkali etching process.463 After the introduction of TiO2, the resultant coating exhibited not only structural color, but antifogging, self-cleaning (superhydrophilic and photocatalytic), and antireflection properties, demonstrating multifunctional characteristics. Grosso et al. demonstrated that hydrophobicity, self-cleaning, antireflection, antifogging, and relatively high mechanical properties can be integrated into a sol−gel coating composed of a methylfunctionalized SiO2 matrix and TiO2 bilayer.464 These multifunctional coatings can be produced in a large scale and exhibit high chemical and mechanical durability, which are promising

Figure 35. Results demonstrating the superhydrophilicity of (7 nm TiO2/22 nm SiO2)6 coated glass (after calcination). (a) Images of a water droplet instantaneously ( OH-terminated > CH3-terminated self-assembled monolayers.475 Ti and its alloys have been widely used in orthopedic and dental implant procedures. After pretreatment with UV illumination, Ti implants showed the superhydrophilic characteristic, resulting in the enhanced strength of bone−titanium integration and increased protein adsorption.476 Superhydrophobic surfaces also strongly affected the protein adsorption. It has been demonstrated that hierarchically structured superhydrophobic surfaces could significantly reduce the adsorption of bovine serum albumin and completely suppress the platelet adhesion and activation.477 The enhanced protein immobilization efficiency was reported on TiO2 surfaces modified with hydroxyl functional groups.478 The hydrophilic property of TiO2 surfaces is dependent on O2 plasma exposure time, owing to the formation of hydroxyl groups on TiO2. The construction of hydroxyl groups with O2 plasma treatment opens an avenue for the development of hydrophilic TiO2 substrates, which has potential in optical immunoassay-based biosensors with a high sensitivity. Cell adhesion and proliferation is an important physiological process, which is strongly affected by surface wettability.479 It was reported that superhydrophobic surfaces can completely inhibit the cell adhesion, while superhydrophilic surfaces will enhance the cell attachment.480 The cell micropattern can be constructed

8.5. Device

Arising from its remarkable chemical and thermal stability, biocompatibility, as well as the environmentally friendly properties, TiO2 is a promising candidate for the design of high-performance devices. Now, many different approaches have been developed to design and construct TiO2-based devices, exhibiting a variety of interesting applications, in the fields of drug delivery, biosensors, microreactors, mass spectrometry, liquid chromatography, artificial ion nanochannels, solar cells, etc. Drug delivery systems represent a vital research area, which is highly interdisciplinary.495 The development of society and 10074


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TiO2 has a strong potential for the design and fabrication of simple and sensitive biosensors. A stable, label-free optical interferometric biosensor was constructed by Choi et al. using TiO2 nanotube arrays, which exhibited superior chemical stability in the pH range 2−12 in comparison with SiO2 and Al2O3-based biosensors.499 Liu et al. investigated the interaction between TiO2 nanoparticles and fluorescently labeled oligonucleotides.500 It was found that TiO2 nanoparticles can be used for the sequence-specific detection of DNA. The integration of drug delivery and bioresponsive detection into one system is important for the development of a smart diagnostic platform. Kong et al. reported the fabrication of mesoporous phosphonateTiO2 nanoparticles, demonstrating simultaneous controlled drug release and bioresponsive sensing.501 The resultant intelligent system can be used as a selective and sensitive platform for target protein detection and drug delivery without other proteins interferences. Recently, mass spectrometry was increasingly used as an approach for the proteomics research. Electrospray ionization or laser desorption/ionization was usually applied to volatize and ionize proteins or peptides for mass spectrometric analysis. In order to avoid the cumbersome washing and polishing procedures, Liu et al. fabricated a TiO2 printed Al foil as the laser desorption/ionization target plate.502 Hydrophilic arrays of TiO2 nanoparticle spots were coated on the Al foil. A mesoporous layer spot was formed through screen-printing or rotogravure-printing followed by calcination, which can be used as an anchor for sample deposition. The TiO2−Al layer exhibited good ability for matrix-free laser desorption/ionization or in situ enrichment of phosphopeptides. Furthermore, the fabrication process is time-saving and cost-effective, which provides a new approach for high-throughput proteomics research. Cheng et al. prepared patterned TiO2 arrays for selective on-plate enrichment and direct matrix-assisted laser desorption/ionization mass spectrometry analysis of phosphopeptides (Figure 37).503 Resorting to the photocatalytical property of TiO2, well-defined and precise patterns were directly produced on OTS-modified films under UV illumination with a photomask followed by NaOH etching. The patterned array possessed hydrophilic TiO2 spots surrounded by the hydrophobic OTS-modified area, which simplified on-target sample preparation and facilitated the enrichment of low-abundance samples. The presented TiO2 array provided a very promising platform for the phosphopeptides analysis (such as human serum, phosphoprotein digests, and nonfat milk samples) by mass spectrometry, demonstrating high selectivity, stability, and reusability. A transparent superhydrophilic−superhydrophobic TiO2 pattern was fabricated by Fujishima et al. through the spincoating of TiO2 films followed by the FAS modification and UV irradiation with a photomask.346 This patterned TiO2 surface with two extreme wetting states can be further site-selectively converted to an alginate hydrogel pattern without appreciable change in the film transparency. Furthermore, the patterned TiO2 film presented promising applications as microreactor arrays. For example, a 50 μm-wide superhydrophilic square can contain about 100 pL aqueous solution. The extremely large wettability contrast makes it possible to confine the aqueous solution within the superhydrophilic area. Owing to the photocatalytic self-cleaning property, the transparent superhydrophilic−superhydrophobic TiO2 pattern could be used as a recyclable pL-level reactor. Biological ion channels that open and close in response to ambient stimuli for the regulation ion permeation through cell

economy resulted in an increased demand for novel and innovative drug delivery technologies to improve the therapeutic efficacy and minimize undesirable side-effects. Recently, TiO2 has received considerable attention in the development of intelligent drug delivery systems.418,496,497 TiO2 nanotube arrays are potential candidates for applications in the drug delivery arising from the combination of geometric features with unique photoinduced properties of TiO 2 . Utilizing a two-step anodization procedure followed by hydrophobic octadecylphosphonic acid modification, Schmuki et al. fabricated amphiphilic TiO2 nanotube arrays (Figure 36).498 The resultant TiO2

Figure 36. (A) Scheme of the procedure for fabricating amphiphilic TiO2 nanotube layers and (B) four methods for drug loading using HRP as a hydrophilic model drug: (I) immersion without any TiO2 surface modification (for reference), (II) immersion after OPDA modification in the upper nanotube layer (hydrophobic cap), (III) covalently attached HRP over the entire nanotube layers, (IV) OPDA cap in the upper nanotube layer and covalently attached HRP in the lower nanotube layer.498 Reprinted with permission from ref 498. Copyright 2009 American Chemical Society.

nanotubes exhibited highly controllable drug release arising from the hydrophobic cap on a hydrophilic TiO2 nanotube. The outer hydrophobic barrier provided an efficient cap against the hydrophilic drug leaching to the aqueous environment. By resorting to the photocatalysis of TiO2 under UV irradiation, the cap can be precisely removed and a highly controlled release of the hydrophilic payload can be achieved. For the biosensor application, an antibody binding with a target molecule of antigen should be immobilized on various substrates using the linker of an amine functional group. Therefore, the surface modification with TiO2 is essential for the biomedical applications. 10075


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fabricated through electrochemical anodization followed by OTS modification. The carboxylic group was introduced on the tip side of TiO2 nanotubes by photocatalytic decomposition of OTS under UV irradiation. When the pore radius of tip side of TiO2 nanochannels was comparable to the thickness of electric double layer, the asymmetric pore geometry in combination with the negatively charged surface resulted in the ion rectification characteristics. The resultant photocatalysis-triggered artificial nanochannel could be used as a diode that rectifies the ion transport, exhibiting potential applications in nanofluid and sensor. Up to now, a variety of TiO2-based devices have been fabricated, which demonstrated a strong application potential. TiO2 is also an interesting candidate for the construction of solar cells, bioimaging, etc. For example, Choy et al. prepared Al-TiO2 composite modified single-layer graphene through the thermal evaporation of thin Al nanoclusters followed by self-assembly of TiO2, which can be used as an efficient transparent cathode for organic solar cells.506 The modified single-layer graphene demonstrated enhanced surface wettability and reduced work function for better energy alignment, simultaneously. The power conversion efficiency of the graphene cathode can reach 2.58% in inverted organic solar cells, exhibiting more efficient performances than those of previously reported. Wu et al. reported the preparation of highly mesoporous TiO2 nanoparticles with a large surface area.507 Further modification with a phosphatecontaining fluorescent molecule and load with an anticancer drug resulted in the intracellular bioimaging and drug delivery properties, respectively.

Figure 37. Schematic diagram of the fabrication of photocatalytically patterned TiO2 arrays for on-plate enrichment and mass spectrometry analysis of phosphopeptides.503 Reprinted with permission from ref 503. Copyright 2011 American Chemical Society.

membranes are important for the implementation of various physiological functions in life processes. Recently, inspired by biological ion channels, a variety of artificial solid-state nanochannels with ion rectification characteristics have been fabricated using different approaches.504 Liu et al. reported photocatalysis-triggered ion rectification in artificial nanochannels using chemically modified asymmetric TiO2 nanotubes.505 Superhydrophobic asymmetric TiO2 nanotubes were

8.6. Liquid Transportation

Directional liquid transport is ubiquitous in nature, which is an important process for biological systems. For example, plant stem,508 cactus spine,44 spider capture silk,43 shorebird beak,509 bat tongue,510 hummingbird tongue,511,512 and other biomaterials have evolved different solutions to achieve liquid transportation. Inspired by these biomaterials, different strategies have

Figure 38. Uncontrollable and unstable liquid droplets on uniform superhydrophobic TiO2 nanotube arrays (a and b). Implementation of adhesion contrast on some typical microdroplet manipulation processes based on the ink-patterned superhydrophobic TiO2 array surfaces: storage (c and d), transfer (e−h), mixing (i and j), and collection (k−m).284 Reprinted with permission from ref 284. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 10076


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can be attributed to the spatial gradient in the surface tension across the droplet, owing to the wettability change along the composite surface. The so-called capillary force overcame droplet pinning and resulted in the droplet transportation.

been developed to fabricate functional materials with liquid transportation.513−515 Usually, in order to transport liquids across solid surfaces, the construction of a gradient in the interfacial tension is critical at the front and rear ends of the droplet acting at the liquid−solid−vapor interface.63,516 Now, the effective control over droplet motion can be achieved through the optimal design surface chemical gradients, physical gradients, or the external field stimuli, showing paramount importance in both fundamental studies and practical applications.517−525 It has been demonstrated that surface tension heterogeneityinduced driving force can be used to guide water motion on flat surfaces. Lin et al. reported unidirectional water-transfer through the construction of a wettability gradient across the fabric thickness.526 The superhydrophobic polyester fabric was prepared using TiO2 and hybrid silica. One side of the superhydrophobic fabric was irradiated subsequently under multiwavelength UV light, resulting in asymmetric wettability through the fabric thickness. The directional water-transfer fabrics induced by an imbalanced surface tension should be able to remove sweat effectively from the body side, which has promising applications in sportswear, soldier’s clothing, etc. Xin et al. reported self-adaptive wettability and unidirectional water permeability using a hydrophilic single-layered cotton substrate with one side possessing superhydrophilicity and the other side possessing hydrophobicity.527 The fibrous textile was treated with TiO2 followed by controlled exposure to “light” and “dark”. Arising from the two extreme wetting states and the cooperation of 2D nanocapillary and 3D marco-capillary effect, water can be transported unidirectionally from the hydrophobic side to the superhydrophilic side. By combining electrochemical anodization with the selfassembly monolayer of POTS, Lai et al. reported the fabrication of superhydrophobic TiO2 patterns with reversible wettability and adhesion.284 It was demonstrated that these superhydrophobic TiO2 arrays have promising applications in droplet manipulation and gas-sensor devices. For example, utilizing the adhesion contrast, droplets were transferred from low adhesive sites to high adhesive target areas (dot-to-dot or dot-to-line), which can be collected by the adhesive guiding track (Figure 38). Furthermore, droplets can roll along the intended route, where the direction to the final target site can be regulated by the adhesion anisotropy of the line. For microfluidic systems, it is important to precisely control the liquid flow within microchannels. Priest et al. reported the photocatalytic lithographic pattern of capillary inner walls to induce specific regions of different wettability and to regulate spontaneous capillary rise.528 First, a homogeneous layer of hydrophilic TiO2 was adsorbed on the capillary wall followed by OTHS modification. The hydrophobic OTHS monolayer was then selectively removed through the TiO2 photocatalytic decomposition under UV irradiation with a patterned photomask. The capillary-driven flow in closed channels can be controlled using well-designed patterned wettability, which has promising applications in microfluidic devices. Monteleone et al. prepared TiO2 nanorods-based composite patterns through photopolymerization.529 The reversible wettability was achieved through the successive storage in vacuum and dark environment. Taking advantage of this property, wettability gradients along the surfaces can be prepared after illuminating adjacent surface areas with increasing time. Lightcontrolled directional droplet movement was demonstrated on TiO2 nanorods-based patterns. This kind of directional motion

8.7. Liquids Separation

Liquids separation, especially oil/water separation is a worldwide challenge arising from the increasing industrial oily wastewater and the frequent oil spill accidents. Learning from nature provides an effective avenue for the development of new technologies for oil/water separation. Recently, a number of functional materials with oil/water separation have been prepared through the rational design of surface structures and surface chemical compositions to achieve desired wettability.530−534 Combining the superwettability conversion with TiO2 photocatalysis, Feng et al. fabricated a double-layer TiO2-based mesh film with superhydrophobicity and superoleophilicity through a hydrothermal method.535 The upper layer is a TiO2 coated mesh film with multiscale structures, while the lower layer is the same TiO2 mesh film with superhydrophobicity and superoleophilicity after ODP modification. The double-layer TiO2-based mesh exhibited the integrated oil/water separation and photocatalytic degradation of soluble pollutants in water under UV illumination (Figure 39). Oils can easily penetrate the mesh film while water stays on the mesh film. Under UV irradiation, the wettability of TiO2-based surfaces was changed from superhydrophobicity to superhydrophilicity arising from the photocatalytic decomposition of ODP by TiO2. Meanwhile, organic pollutants in the water can be degraded and the purified water penetrated the films. Cui

Figure 39. Image of the experimental process of water purification using a double-layer TiO2-based mesh film: (a) methylene blue aqueous solution and petroleum ether stay on the mesh film before separation; (b) only the aqueous solution stays after the separation; (c) device was placed vertically before UV illumination; (d) methylene blue in water was photodegraded after 2 h of UV illumination, and purified water flowed down from the device.535 Reprinted with permission from ref 535. Copyright 2013 Royal Society of Chemistry. 10077


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et al. reported the wettability conversion between superoleophobicity and superhydrophilicity on TiO2/SWNT composite coatings under UV irradiation.374 Utilizing this property, a TiO2/SWNT coated stainless steel grid can be used to separate a mixture of liquids with different surface tensions. For example, a mixture of silicone oil (21.5 mN/m) and 40% ethanol (30.2 mN/ m) can be separated through the grid using this approach. Silicone oil droplets can easily penetrate the grid, whereas 40% ethanol mixed in the silicone oil beads up on grid surfaces. Using the same grid, diiodomethane (50.8 mN/m) and glycerol (63.0 mN/m) can be separated through further increasing the surface energy of the coating under UV light irradiation. The composite coatings possessed stable wettability, which was still effective in liquid separation after exposing in air for more than 90 days. The conventional strategy for oil/water separation is dependent on the superwetting materials with simultaneous superhydrophobicity and superoleophilicity. Recent research works demonstrated underwater superoleophobic surfaces can be used in the field of oil/water separation. Zeolites are an important class of inorganic materials with crystalline nanoporous structures, possessing wide applications in adsorption, catalysis, and ion exchange.536−540 Recently, Yu et al. first reported zeolite-coated mesh films with superhydrophilicity and underwater superoleophobicity for efficient gravity-driven oil/water separation.541 The flux and intrusion pressure can be regulated during the oil/ water separation through the adjustment of pore size, which is dependent on the crystallization time of zeolite. The low adhesive underwater superoleophobicity prevented the zeolitecoated films from fouling by oils, showing long-term recyclability. Furthermore, arising from the superior thermal, chemical, and mechanical stability of zeolites, these films showed excellent corrosion resistance even in the corrosive media, which have promising applications in practical oil/water separation. The purification of industrial and agricultural sewage containing harmful organic molecules is a worldwide challenge, arising from its effect on the environment and human health. A facile and versatile approach was developed by Yu et al. to achieve highly efficient water purification.542 Flexible TiO2−SiO2 composite nanofibrous membranes with hierarchical porous structures were fabricated using the electrospinning method. The resultant superhydrophilic TiO2−SiO2 membrane presented high adsorption capacity and permeability during the purification of methylene blue solution (Figure 40), owing to the superwettability and hierarchical porous structures with intrafiber and interfiber pores. These membranes with superior recyclability also have great promise for the removal of heavy metal ions in polluted water. Recently, Miyake et al. fabricated an underwater superoleophobic TiO2 surface by calcining the Ti mesh with a pore size of approximately 150 μm.388 The obtained TiO2 mesh can be used as an oil/water separation filter (Figure 41). After UV irradiation, the TiO2 mesh was dipped into deionized water to form a thin water film on its surface. Water can quickly permeate through the mesh, while oils were retained on the mesh. This oil/ water separation property can be ascribed to the underwater superoleophobicity and almost no adhesion toward the oil droplet underwater. After the storage in dark, the superwettability can be recovered under UV irradiation, demonstrating the high capacity, durability, antifouling, and recyclability as an oil/water separation filter.

Figure 40. (a) Typical photographic sequences of 3 μL water droplet spreading on superhydrophilic TiO2−SiO2 composite membranes with ultrafast spreading property. (b) Illustration of the permeation and adsorption process: The methylene blue solution spreads rapidly on the membrane surface and permeates through the interspace between nanofibers in the membrane, meanwhile methylene blue molecules are predominantly captured by intrafiber pores, and then purified water is obtained.542 Reprinted with permission from ref 542. Copyright 2013 Royal Society of Chemistry.

8.8. Offset Printing and Liquid Reprography

Today, offset printing is a commonly used technique for printing of advertising leaflets, magazines, and books. With the economic development and diversified needs, the current printing technology is faced with many challenges for the improved printed product quality and high productivity. Many different synthesis strategies have been developed to simplify the platemaking process and enhance the resolution of the offset printing. Since surface wettability plays an important role in printing and reprography, a great number of studies have been focused on the precise control of liquid pattern on solid surfaces with superwettability. Recently, Fujishima et al. reported a series of interesting works on offset printing using TiO2-based superhydrophilic−superhydrophobic patterns.355−358 For example, a facile and novel fabrication process was developed to produce superhydrophobic-superhydrophilic patterns via an inkjet technique on anodized Al, which can be used as the substrate for offset printing.355 During the offset printing, the superhydrophobic area repelled moistening water, while the superhydrophilic regions were wetted. Finally, the ink pattern was formed on the plate. Color printing with a resolution of 133 lines per inch was successfully fabricated using this strategy. Furthermore, these TiO2-based printing plates were reusable. After the printing, the remaining ink on the surface can be removed through the TiO2based self-cleaning effect under UV irradiation and the surface was converted to superhydrophilicity again. After the repetition of ODP modification, the repatterned TiO2-based plates can be used to print a second image, exhibiting the reusability of the printing plate. A web press demonstrated that more than 5000 pages can be printed with a maximum printing speed of 40 000 copy h−1 using the TiO2-based printing plate. This indicated that TiO2-based superhydrophilic−superhydrophobic patterns possessed practical applications in offset printing. A similar synthesis strategy has also been developed to prepare TiO2-based superwetting patterns on Ti substrates.356 The resultant Ti substrate with superhydrophilic−superhydrophobic patterns can be used as an offset printing plate with high resolution (133 and 10078


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Figure 41. Schematic diagram and photographs of the oil/water separation device containing the TiO2 mesh films with a pore size of about 150 μm. nHexadecane dyed red and water dyed blue were used as the oil and water, respectively, in these photographs.388 Reprinted with permission from ref 388. Copyright 2013 American Chemical Society.

Figure 42. Schematic diagrams of the liquid reprography process. (a) The surface wettability behaves as a Cassie superhydrophobic state below the electrowetting threshold voltage. (b) The surface wettability locally changes from the Cassie to the Wenzel state under the patterned-light “▲” illumination on the aligned, composite nanopore array below the electrowetting threshold voltage. (c) After the light and the voltage are turned off, the ink pattern “▲” is left on the removal of redundant ink. (d) When the ink pattern is transferred to printing paper, the desired image, “▲”, is obtained.545 Reprinted with permission from ref 545. Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

magnetic field, pH, photoelectric cooperation, temperature, enthalpy, selected solvent and other stimulus through the combination of optimal surface geometrical morphology and surface chemistry. Among the wide variety of external stimulus developed so far, light and photoelectric cooperative wettability is one of most promising, which can be used to achieve liquid patterns for printing. Recently, Jiang et al. reported the patterned wetting transition from the Cassie state to the Wenzel state on superhydrophobic aligned ZnO nanorod arrays through a photoelectric cooperative wetting approach.543 Liquid reprography was achieved using the patterned wettability transition under patterned-light irradiation. However, the aligned ZnO array structure was easily damaged arising from the low mechanical strength and could not sustain multiple transfer

150 lines per inch), which were renewable after UV irradiation and resurfacing. In comparison with the initial image, the second printed image with the renewed substrate exhibited no significant difference in the image quality. It was found that 1000 sheets of high-grade paper can be printed at a rate of 4000 sheets per hour by a sheet offset press. Furthermore, the Ti-based offset printing plate showed strong adhesion toward the TiO2 layer on the Ti substrate and long plate life in comparison with Al substrates. This proposed strategy provided a promising avenue for the resource-saving and environmentally friendly printing. Materials surfaces with single wettability cannot meet the demand in the rapid development of printing techniques. In the past few years, stimuli-responsive wettability has been intensively investigated under external stimulus, such as light, electric field, 10079


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printing, which is a nuisance in the practical application. Recently, a similar work was reported for the fabrication of photoelectric cooperative wettability using the superhydrophobic CdS quantum dot photosensitized TiO2 nanotube arrays.544 Owing to the introduction of CdS quantum dots, the modified TiO2 nanotube surface demonstrated improved photosensitivity in the visible light region. Indium tin oxide glass was used as top electrode to apply electrical and optical stimuli. Patterned wettability conversion was achieved under light irradiation on the top electrode, which can be used in liquid reprography. In order to improve the mechanical strength of nanostructures and the controllability of patterned wettability, Jiang et al. also fabricated superhydrophobic aligned arrays using TiO2-coated nanoporous anodic aluminum oxide films.545 After photosensitizing with a dye and modification with low surface energy materials, photoelectric cooperative induced patterned wettability was achieved on the array surface (Figure 42). The well-knit and uniform array demonstrated the robust and reusable printing owing to its high mechanical strength. Liquid reprography developed using the patterned wetting strategy on the superhydrophobic TiO2-coated array surface opens a new progress in liquid reprography and will gear up its practical application.

Figure 43. Schematic diagram of the decontamination process occurring on the superhydrophilic self-cleaning surface.554 Reprinted with permission from ref 554. Copyright 2006 Elsevier.

In order to fabricate TiO2-based materials with both superhydrophilicity and self-cleaning under UV irradiation, different synthesis strategies have been proposed in recent years.241,555−559 Fujishima et al. fabricated superhydrophilic core−shell-like TiO2−SiO2 particle coatings by the stepwise electrostatic deposition.241 The resultant coating exhibited both self-cleaning and antireflection properties. The submicrometersized SiO2 particle single layer provided a porous structure with low refractive index, which favors the antireflection effect. Nanosized TiO2 particles on the SiO2 layer resulted in superhydrophilicity and self-cleaning under UV irradiation. Although TiO2 particles possessed a higher refractive index than the glass substrate, TiO2−SiO2 coatings enhanced the maximum transmittance of the glass to greater than 99% owing to their special surface structures. Using a sol−gel dip-coating method, Fujishima et al. also fabricated SiO2−TiO2 bilayer films with self-cleaning and antireflection.557 After UV irradiation, the bilayer films exhibited superhydrophilicity, which favored the self-cleaning function. Inspired by the insect compound eye, Fujishima et al. reported self-cleaning and antireflective PET films with a moth-eye-like surface.558 PET films coated with TiO2 particles possessed a high transmittance and almost no absorption in the visible light range, resulting in the antireflection. Furthermore, the resultant film also exhibited superhydrophilicity and self-cleaning under UV irradiation. Since the discovery of graphene in 2004, graphene was considered as a rapidly rising star material and attracted a great deal of attention owing to its unique properties.560−562 Miyauchi et al. prepared graphene-loaded TiO2 thin films for self-cleaning applications on glass substrates through the spin-coating approach.559 The resultant film exhibited high transparency, electroconductivity, photocatalytic activity, and photoinduced superhydrophilicity. The photocatalytic oxidation and photoinduced superhydrophilicity have been enhanced, arising from the efficient charge separation in TiO2 through electron transfer from a conduction band of TiO2 to graphene. The electroconductivity of graphene− TiO2 films also contributes to the self-cleaning owing to its antifouling effect against particulate contaminants. In order to extend the applications of TiO2 in self-cleaning, it is essential to fabricate superhydrophilicity-induced self-cleaning TiO2 surface without UV irradiation or under visible-light irradiation. Diau et al. prepared anatase nanotube arrays on a Ti foil or on a glass substrate in fluoride ion containing sulfate electrolytes.563 The resultant nanotube arrays exhibited a superhydrophilic self-cleaning property without illumination, which can be attributed to the capillary effect. Tan et al. reported transparent visible light activated C−N−F-codoped TiO2 films with self-cleaning through a layer-by-layer dip-coating approach.564 The C−N−F-codoped TiO2 film possessed strong

8.9. Self-Cleaning

Self-cleaning is a desired property for human beings, which enables glass, textiles, ceramic tiles, woods, plastics, and buildings to repel water, bacteria, dirt, and grime. This will not only save maintenance time and costs but reduce water and chemical use, contributing to the ecosystem. In the past few years, self-cleaning materials have attracted much attention owing to their important advantages in industry, agriculture, military, and daily life.5−7,47,48,68,75,546−549 Nowadays, a variety of self-cleaning materials have been commercialized. On the basis of the selfcleaning mechanism, four categories can be classified,6 (i) TiO2based self-cleaning resulting from the photocatalysis and photoinduced superamphilicity; (ii) superhydrophobicity-induced self-cleaning similar to the natural lotus effect; (iii) gecko setae-inspired dry self-cleaning; and (iv) underwater organisms-inspired antifouling self-cleaning (pilot whale skin, shark skin, carp scale et al.). In this section, we will only focus on the application of TiO2-based materials in self-cleaning, particularly on TiO2-based superhydrophilic or superhydrophobic surfaces. 8.9.1. Superhydrophilicity/Photocatalysis-Induced Self-Cleaning. In nature, a variety of biological materials surfaces exhibited superhydrophilicity. Water droplets can spread out and form a thin water film on the superhydrophilic surface. When water flows on these surfaces, it tends to spread out and then takes dusts away, showing the self-cleaning ability. Among the wide variety of superhydrophilic artificial materials, TiO2 is one of most promising. The simultaneous photocatalysis and photoinduced superamphilicity of TiO2 results in the desired self-cleaning characteristic, which endows TiO2 with a wide variety of practical applications. For the TiO2-based self-cleaning materials, TiO2 can decompose organic contaminants or kill bacteria adhering on its surface under UV or visible irradiation. Moreover, the superhydrophilicity of TiO2-based materials favors the fast and complete spreading of water droplets on their surfaces, which also contribute to the removal of contaminants (Figure 43). In the past decades, a series of TiO2-based self-cleaning coatings have been fabricated on a variety of substrates.5−7,550−553 10080


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Figure 44. Applications of self-cleaning exterior building materials. (a) China’s new National Opera Hall features self-cleaning glass coated with a film of photocatalytic nanoparticles that can break down dirt.569 Reprinted with permission from ref 569. Copyright 2005 American Association for the Advancement of Science. (b) Dives in Misericordia, a church constructed of TiO2-containing self-cleaning cement, in Rome.568 Reprinted with permission from ref 568. Copyright 2004 Cambridge University Press. (c) Eco-life-type houses using self-cleaning tiles and glass in Japan.5 (d) Selfcleaning roof of a train station in Motosumiyoshi (courtesy of Taiyo Kogyo).5 Reprinted with permission from ref 5. Copyright 2008 Elsevier.

the surface.78 Therefore, the most efficient use of TiO2 coatings should be as exterior architectural materials, which can be expose to abundant sunlight and natural rainfall. Since the late 1990s, a variety of TiO2-based self-cleaning materials have been commercialized, including tiles, aluminum siding, glass, tent materials, plastic films, cement, and others.5,546,568 For example, in China, self-cleaning glass was coated with TiO2 nanoparticles, which has been used on the surface of National Opera Hall (Figure 44a).569 A white cement consisting of TiO2 nanoparticles has been applied for the construction of the Dives in Misericordia Church in Rome (Figure 44b).568 In Japan, a great number of buildings have been covered with self-cleaning tiles containing TiO2. Since 2003, the PanaHome Company, one of the major house manufacturers in Japan, has marketed “eco-life”-type houses. Self-cleaning windows and tiles as well as a solar cellcovered roof-top were utilized (Figure 44c).5 Self-cleaning TiO2 materials have also been applied on the roof of a train station in Motosumiyoshi in Japan (Figure 44d). 8.9.2. Superhydrophobicity-Induced Self-Cleaning. After billions of years of evolution, some biological surfaces exhibit superhydrophobicity-induced self-cleaning. 29,47,68 Among the variety of natural self-cleaning materials, the lotus (Nelumbo nucifera) leaf is one of most promising. In Asia, lotus has been a symbol of purity religions and cultures because of its self-cleaning nature for over 2000 years. It has been demonstrated that the well-known lotus effect is attributed to the cooperation of surface randomly distributed micronanoscale structures and surface hydrophobic epicuticular waxes, resulting in superhydrophobicity with a low water sliding angle (Figure 16). Water droplets on the lotus leaf surface are almost spherical and can roll freely in all directions and then pick up dirt particles. In order to generate the superhydrophobicity-induced selfcleaning, both a high static water contact angle and a low contact angle hysteresis are essential.

visible-light absorption and enhanced photocatalytic activity for stearic acid decomposition under visible light irradiation, resulting from the synergetic effect of the doped C, N, and F atoms and a high surface area. Furthermore, these films also demonstrated nonlight activated superhydrophilicity. This should be ascribed to the high surface roughness which hindered the conversion from hydrophilicity to hydrophobicity in the dark and resulted in the superhydrophilicity without light irradiation. Recently, Fu et al. fabricated multiscale flake-like Bi2MoO6/TiO2 bilayer films using a solvothermal process. These flake-like films showed visible-light-induced self-cleaning properties.565 This can be attributed to the cooperation of superhydrophilicity, surface hierarchical structures, and enhanced charge separation efficiency at the heterojunction interfaces. The resultant Bi2MoO6/TiO2 films should have promising applications in photovoltaic cells and photoelectrochemical sensors. In recent years, demands for a green and comfortable lifestyle make it desirable to design self-cleaning fabrics through the TiO2 immobilization on fabric surfaces without a detrimental influence on the character of textile fabrics. Pakdel et al. reported the modification of wool fabrics using TiO2−SiO2 nanocomposite by a low temperature sol−gel method.566 The resultant wool fabrics exhibited superhydrophilicity and self-cleaning. Li et al. fabricated self-cleaning cotton fabrics using an esterification reaction followed by cograft polymerization of 2-hydroxyethyl acrylate together with TiO2 under γ-ray irradiation.567 The functionalized cotton fabrics exhibited enhanced hydrophilicity in comparison with the pristine cotton fabric. Under UV irradiation, these cotton fabrics exhibited a photocatalytic selfcleaning effect. An accelerated laundering durability test indicated that covalent bonds between TiO2 and cotton fabrics can survive 30 accelerated laundering circles, which is equivalent to 150 instances of commercial or domestic launderings. It was proposed that the self-cleaning property of TiO2 can be improved when water flow, such as natural rainfall, was applied to 10081


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al. fabricated superhydrophobic photocatalytic surfaces by direct incorporation of TiO2 nanoparticles into a polymer matrix (Figure 45b).573 Even after long-term UV irradiation, the polymer embedded TiO2 films still exhibited both superhydrophobicity and photocatalysis simultaneously. This is the first report of a surface possessing dual functionality after extensive UV irradiation, showing promising applications as commercial self-cleaning films. The simultaneous superhydrophobicity and photocatalyticity of TiO2 nanoparticles embedded polymer films can be attributed to the following consideration, (i) the exposed surfaces consist mainly of the polymer matrix, which prevent a significant reduction of surface hydrophobicity upon UV irradiation, and (ii) the PDMS-based polymer matrix can resists photocatalytic breakdown caused by the embedded TiO2 nanoparticles.

Through a layer-by-layer deposition process followed by the heat treatment and PTES modification, Rohani et al. fabricated a superhydrophobic coating on steel substrates.570 The resultant superhydrophobic surfaces exhibited anticorrosion, UV resistance, and long-term stability even to the acid solution, which have promising applications in harsh environments such as high temperature, high UV irradiation, and outdoor applications. Utilizing the hydrothermal synthesis strategy, Qi et al. prepared rutile TiO2 nanorod arrays with parallel nanotips grown on the top of individual nanorods.571 The resultant TiO2 arrays exhibited broadband and quasi-omnidirectional antireflection. Furthermore, the obtained TiO2 arrays exhibited high chemical and thermal stability as well as self-cleaning, showing potential applications in chemical sensors and solar cells with improved efficiency. In order to fabricate superhydrophobicity-induced selfcleaning materials, a variety of fluorinated polymers have been used to decrease the surface free energy. The design and construction of superhydrophobic surfaces using nonfluorinated polymers is an environmentally friendly approach, which has received much attention in recent years. Recently, Zhang et al. reported superhydrophobic TiO2 nanowire coatings using the low surface energy PDMS modification (Figure 45a).572 The

8.10. Site-Selective Functional Patterning

Superhydrophilic−superhydrophobic TiO2 patterns simultaneously possess two extreme wetting states. Utilizing the difference in surface wettability, it has demonstrated that these functional surfaces with patterned wettability can be used as templates for the design and fabrication of site-selective structured patterns. Lin et al. reported superhydrophilic−superhydrophobic TiO2 patterns through the electrochemical anodization approach combined with POTS modification and UV irradiation with a photomask.349 Utilizing the resultant TiO2 superwetting patterns as templates, different synthesis strategies have been proposed to fabricate well-defined site-selective patterns, such as octacalcium phosphate,349,574 calcium phosphate,351 Ag nanoparticles,351 vertically aligned TiO2 nanotube array,575 ZnO−TiO2 nanostructures,576 and cell-and-protein micropatterns.577 For example, using the high contrast wetting TiO2 pattern as a template, bioactive 3D calcium phosphate patterns were prepared through the electrochemical deposition.351 These calcium phosphate patterns exhibited a high site-selective manner, which can be further used in cell immobilization and other biomedical fields. Patterned superhydrophilic−superhydrophobic templates provide a promising avenue to design and construct micro- and nanodevices with potential applications in microfluid and biotechnology. As discussed above, TiO2 possesses photoinduced superhydrophilic and photocatalytic properties, which makes it possible for the fabrication of TiO2-based superhydrophilic− superhydrophobic patterns. The unique photochemical property of TiO2 provides a novel approach for the easy and precise construction of structured arrays on TiO2 surfaces with patterned wettability. For example, Sato et al. reported the patterning of a colloidal crystal film on superhydrophilic−superhydrophobic patterned TiO2 surfaces.578 Under UV illumination with a photomask, TiO2 surfaces with patterned wettability were fabricated through the combination of the sol−gel method and postmodification with FAS. A patterned colloidal crystal film was formed after immersing the substrate with superwettability patternings into an aqueous suspension consisting of monodispersed silica spheres or polystyrene. The liquid surface is convex over the hydrophobic area, while it is concave above the hydrophilic site. Furthermore, a complex pattern composed of monodispersed spheres with different diameters can be prepared by repeating the processes of fluoridation and photodecomposition. The presented approach for the preparation of patterning structured arrays makes it possible for the practical application of colloidal crystals as optical devices and pigments.

Figure 45. (a) Self-cleaning function superhydrophobic surface based on TiO2 nanowires combined with polydimethylsiloxane using graphite powder as contaminants.572 Reprinted with permission from ref 572. Copyright 2013 Elsevier. (b) Side-on SEM image of a polymer film with embedded anatase TiO2 nanoparticles. The deposition used a substrate temperature of 360 °C and a total of 0.1 g of nanoparticles in the precursor. Inset: a 3 μL water droplet sitting on the surface of the composite film.573 Reprinted with permission from ref 573. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

resultant material exhibited superhydrophobicity-induced selfcleaning and the antifouling performance for organic solvents, which can self-remove the organic solvents layer and recover its original superhydrophobic behavior. Although a great number of superhydrophobic biomaterials can be found, there are no examples of superhydrophobic surfaces that simultaneously act as photocatalysts in nature. Recently, utilizing the aerosol assisted CVD approach, Parkin et 10082


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Figure 46. Fabrication procedure of microelectrode arrays: (a) superhydrophobic ODS-modified TiO2 film, (b) TiO2 nanoparticle-based microelectrode arrays after photolithography with a photomask, and (c) metal nanoparticle-based microelectrode arrays after site-selective photocatalytic deposition of metal nanoparticles.579 Reprinted with permission from ref 579. Copyright 2009 American Chemical Society.

et al. fabricated low adhesive superhydrophobic TiO2 surfaces through a combination of surface roughing and ODP modification.345 Under UV irradiation with a photomask, the photocatalytic decomposition of ODP resulted in the formation of superhydrophilic−superhydrophobic patterns on TiO 2 surfaces. It was found that these patterns could guide the water condensation and evaporation of polystyrene microsphere suspensions, arising from the extremely large wettability contrast between superhydrophobic and superhydrophilic sites. For example, during cooling at 5 °C with 60% relative humidity, water selectively condensed in the superhydrophilic stripes to form first a water droplet and then a water column in sequence (Figure 47). A water bulge was also observed in the superhydrophilic stripes. This indicated superhydrophilic stripes have an effect on water condensation on superhydrophobic surfaces. Water droplets formed on the superhydrophobic area can spontaneously move to the superhydrophilic domain, which will be confined as they touched the border of the two areas.

With the development of nanoscience and nanotechnology, some microelectrode arrays as important analytical tools have been fabricated using nanomaterials, demonstrating enhanced properties in comparison to classical macroelectrodes. Utilizing the photolithography and the site-selective photocatalytic deposition of superhydrophobic n-octadecyltriethoxysilanemodified TiO2 films, Tian et al. fabricated TiO2 microelectrode arrays with superhydrophobic-superhydrophilic patterns (Figure 46).579 Furthermore, the resultant TiO2 patterns can be used as molecular microtemplates to selectively fill superhydrophilic area with water-soluble materials and construct metal nanoparticlebased microelectrode arrays (such as gold nanoparticle-based patterns). These microelectrode arrays exhibited the enhanced analytical performance, showing their potential for the development of new analytical devices. 8.11. Water Condensation

It has been demonstrated that CF4 plasma is a versatile approach to roughen the TiO2 surface.303 Utilizing this strategy, Fujishima 10083


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efficiency through the combination of TiO2 photocatalysis and constructed wetlands.592 The combined system could enhance the treatment efficiency of polluted water and remove disinfection byproduct. The reclaimed water met the maximum contamination levels of drinking water standards, which showed promising applications for the treatment of agricultural and domestic wastewater. It was also reported that TiO2 nanoparticles can enhance the light absorbance, accelerate the nitrogen metabolism, and then promote plant growth.593−595 Over the past decade, the increasingly environmental problems, especially water pollution and air pollution, have received considerable attention. A variety of solutions for environmental protection have been proposed to achieve both socioeconomic and environmental sustainability. As one of the most viable environmental cleanup technologies, TiO2 photocatalysis has attracted much attention in air purification, water purification, and other environment-related fields. Contaminants indoors, such as VOC, bacteria, fungi, etc. threaten human health. TiO2 was proved to be versatile to photodegrade pollutants and to kill bacteria for indoor air purification.596−600 Operando IR spectroscopy was used by ElRoz et al. to investigate the effect of parameters, such as temperature, UV irradiation intensity, and VOC concentration on photocatalytic activity and selectivity of TiO2 nanotubes in air purification.601 It was found that the synthesis parameters for TiO2 nanotubes could affect the photocatalytic activity in the air purification process.602 Wang et al. reported the fabrication of Pt/N-TiO2 photocatalysts for indoor air purification under room light, demonstrating effective degradation of VOC.603 Vorontsov et al. found nanosized TiO2 aerosol can be used to remove diethyl sulfide vapors from air through the adsorption and photocatalysis.604 Economic growth accompanying environmental pollutions resulted in increasing water pollution and dwindling water quality around the globe. Drinking water scarcity has been identified as a major global problem in the 21st century. Therefore, water purification was identified as one of the most important technologies. Arising from its strong oxidation ability, TiO2 is expected to be a promising candidate for water purification.205,210,581−584 During the TiO2 photocatalytic decontamination process, no toxic intermediate products are generated, which is important for water purification and reclamation.605−607 Hydrophobic membranes have promising applications in membrane distillation for water purification. Mansouri et al. fabricated superhydrophobic membranes for the application in distillation.608 A low temperature hydrothermal method was developed to construct multiscale structures through depositing TiO2 nanoparticles on microporous PVDF membranes. After surface fluorosilanization, the resultant TiO2 coated membranes exhibited both superhydrophobicity and superoleophobicity. The liquid entry pressure of water increased from 120 to 190 kPa arising from the change in the surface chemistry of the membrane. Compared to the virgin membrane, a significantly higher flux recovery was found for the superhydrophobic membrane. Recently, an O3-assisted photocatalytic water-purification unit was developed by Fujishima et al. using a TiO2 modified Ti mesh filter.609 The developed water purification system is composed of a water-purification unit, a pump, a reservoir, and an O3 production unit, exhibiting longterm stability and high degradation efficiency for biological and chemical contaminants (Figure 48). This system showed potential applications in sewage water treatment.

Figure 47. (a−c) Optical micrographs of a superhydrophobicsuperhydrophilic pattern cooled below the dew point with a cooling/ heating stage for 10 s (a), 3 min (b), and 14 min (c). (d) Micrograph of a superhydrophobic surface cooled below the dew point for 3 min. The scale bars in the micrographs correspond to 100 μm.345 Reprinted with permission from ref 345. Copyright 2007 American Chemical Society.

These superhydrophilic−superhydrophobic patterns exhibit promising applications in inkjet printing, microreactors, fluid microchips, etc. 8.12. Agricultural and Environmental Fields

In the past few decades, water and air pollution have received the global attention, which is a serious threat to the environment and human health. For example, the large-scale developments of agrochemical and pharmaceutical industries as well as the intensive use of chemicals have resulted in the worldwide pollution in the agriculture and environment. A variety of approaches have been proposed to solve the major global problem of the 21st century. Some recent works have established that TiO2 has important applications in agricultural and environmental fields, arising from its outstanding physical and chemical properties.5,205,210,580−585 In the field of agriculture, TiO2-based materials exhibited promising applications in pesticides, pollutant degradation, bactericidal, water purification and reclamation, etc.580 Usually, conventional pesticides applied directly to soils, turf, or plants will resulted in the pollution of agricultural soil, crops, freshwater, watercourses, and aquatic organisms. TiO2 nanomaterials were proved to be essential for the design of nanopesticides in agricultural production and crop protection.586 Moreover, plant surfaces after hydrophobic modification can resist a range of agricultural insect pests.587 In addition to its use for nanopesticides, TiO2 also can be applied for the pesticide degradation.588 For example, Mahmoo di et al. reported that photocatalysis using immobilized TiO2 nanoparticles was an effective avenue for the degradation and mineralization of pesticides such as Butachlor, Diazinon, and Imidacloprid from contaminated water.589,590 Fusarium is a common plant disease, which could result in the decrease of grain quality/yield and the food safety due to the generation of mycotoxins. Li et al. fabricated Pd-modified N-doped titanium oxide nanoparticle photocatalysts, which exhibited effective antifungal activity to Fusarium graminearum under visible light irradiation.591 The photocatalytic disinfection mechanism can be attributed to the cell wall/membrane damage caused by the attack from reactive oxygen species. Chen et al. investigated the water reclamation 10084


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practical applications of TiO2-based materials with superwettability, the following research directions should be addressed. (a) Optimization of bioinspired TiO2 materials with special wettability requires a detailed knowledge of the structure− property relationships and a deeper understanding of their formation mechanisms. In the past few years, the continuing breakthroughs in the successful synthesis and modification of TiO2-based materials are the proof of the deeper knowledge and better design techniques available, which bring new functions and new applications with enhanced performance. However, further fundamental investigations are still necessary. For example, deeper understandings on the photoinduced superhydrophilic mechanism of TiO2 rutile (110) surfaces through the experimental and theoretical approaches would be useful to develop a general mechanism for TiO2 materials and to design novel advanced TiO2-based materials. On the other hand, interdisciplinary cooperation is still necessary to further reveal the relationship between multiscale structures and special surface wettability of natural materials. This would be helpful to extract novel engineering principles and to develop new technology for practical applications. (b) Self-healing is an inherent characteristic for living organisms. Biomaterials can maintain their special wettability by regenerating their epicuticular wax layer when damaged. In order to stimulate the applications of TiO2 materials, it is essential to design and construct smart TiO2 materials with selfhealing, which can spontaneously recover their structure and superwettability when damaged. This should be a research focus in the near future. Considering the practical application of TiO2based films with special wettability, sufficient mechanical and thermal stability should also be an ongoing endeavor. (c) The current research on superwetting TiO2 materials is mainly focused on the solid−liquid−vapor three-phase system. Recently, underwater superwettability (especially underwater superoleophobicity) of a solid surface has attracted much attention owing to its important practical applications. Although this research direction is in its infancy, it is a rapidly growing and promising research direction, which strongly extends the research field of special wettability to the liquid−liquid-solid system. Therefore, the surface wettability of a TiO2 film in the liquid−liquid-solid system should also be an interesting research focus in the near future. (d) Through evolution, nature has learned what is optimal.618 In view of the commercial applications, the construction of multifunctional TiO2 materials is an inevitable trend in the future. Bioinspired strategies are expected to be particularly effective. Multifunctional integration is an inherent and efficient characteristic for biomaterials. Therefore, optimized biological solutions should provide some important inspiration for scientists and engineers to design advanced TiO2 materials with multifunctional integration. This would be helpful to solve the growing global energy and environmental crisis to a great extent.

Figure 48. Schematic view of a water-purification system using the ozone-assisted photocatalytic water-purification unit.609 Reprinted with permission from ref 609. Copyright 2012 Royal Society of Chemistry.

Another important application of TiO2 in water purification is the removal of toxic inorganic species from water. For example, arsenic contamination via groundwater has become a serious global environmental problem. As(III) is more toxic than As(V). Therefore, the oxidation of As(III) to As(V) is desirable in water treatment. In a few decades, many different technologies have been developed to remove arsenic from water.610 TiO2 is a promising candidate for arsenic removal from water, arising from its negligible toxicity, strong oxidation ability, and adsorption property. A number of efforts were devoted to fabricating TiO2based materials and to investigating their applications in the removal of inorganic and organic arsenic from water.611 Rajeshwar et al. reported that photogenerated holes of TiO2 can oxidize As(III) to As(V) under UV light irradiation.612 Subsequent deeper understandings on kinetics and mechanisms of TiO2 photocatalytic oxidation of arsenite strongly accelerated the technological development of water purification.613−615 Fu et al. fabricated mesoporous TiO2/α-Fe2O3 composites through the impregnation of Fe3+ into mesoporous TiO2.616 The resultant composites exhibited simultaneous photocatalytic oxidation of As(III) to As(V) and As(V) adsorption, showing promising applications in water treatment. Choi et al. investigated the photooxidation of arsenite under 254 nm UV irradiation, where arsenite in water was oxidized to arsenate in the absence of any chemical reagents.617 The presence of dissolved oxygen resulted in the improved photooxidation rate of As(III). The dissolved oxygen played a dual role as a precursor of reactive oxygen species and an electron acceptor, where superoxide, H2O2, and OH radicals were all generated in situ, resulting in the overall quantum yield higher than 1.

9. CONCLUSION Bioinspired TiO2 materials with special wettability constitute an important domain in materials chemistry, which is experiencing booming development. In the past few years, many different synthesis strategies have been developed to design and fabricate bioinspired TiO2 materials with special wettability. These materials exhibit practical applications in the fields of antibacteria, anticorrosion, antifogging, biomedical, device, liquid transportation, liquids separation, offset printing, liquid reprography, self-cleaning, site-selective functional patterning, water condensation, agriculture, etc. which also provide a promising solution to solve the growing global environmental pollution and damage to a great extent. In order to further promote the

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 10085


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Biographies

Akira Fujishima was born in 1942 in Tokyo. He received his Ph.D. in Applied Chemistry at the University of Tokyo in 1971. He taught at Kanagawa University for four years and then moved to the University of Tokyo, where he became a Professor in 1986. In 2003, he retired from this position and took on the position of Chairman at the Kanagawa Academy of Science and Technology. From 1st of January 2010, he became President of Tokyo University of Science. His main interests are in photocatalysis, photoelectrochemistry, and diamond electrochemistry.

Kesong Liu received his Ph.D. degree (2006) in materials science at Harbin Engineering University under the supervision of Prof. Honggang Fu and Prof. Milin Zhang. He then worked as a postdoctoral fellow (2006−2008) in Prof. Lei Jiang’s group at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS). He is currently an associate professor at the School of Chemistry and Environment, Beihang University (BUAA). His research interests are focused on the design, construction, and application of bioinspired materials with multifunction integration.

Lei Jiang received his Ph.D. degree (1994) from Jilin University in China (Tiejin Li’s group). He then worked as a postdoctoral fellow in Prof. Akira Fujishima’s group at Tokyo University. In 1996, he worked as a senior researcher in the Kanagawa Academy of Sciences and Technology under Prof. Kazuhito Hashimoto. In 1999, he joined ICCAS as part of the Hundred Talents Program. In 2009, he was elected academician of the Chinese Academy of Sciences. He is currently a professor at ICCAS and Dean of the School of Chemistry and Environment, Beihang University. His research interest focuses on bioinspired interfacial functional materials.

ACKNOWLEDGMENTS We appreciate the financial support of National Natural Science Foundation of China (21273016, 21121001, and 91127025), the National Basic Research Program of China (2013CB933003, 2011CB935700, and 2010CB934700), Australian Research Council Discovery Project (DP140102581), the Program for New Century Excellent Talents in University, Beijing Natural Science Foundation (2122035), Beijing Higher Education Young Elite Teacher Project, the Fundamental Research Funds for the Central Universities, and the Key Research Program of the Chinese Academy of Sciences (KJZDEW-M01).

Moyuan Cao is currently a Ph.D. candidate at the School of Chemistry and Environment, Beihang University under the supervision of Prof. Lei Jiang and Assoc. Prof. Kesong Liu. He received his B.Eng. degree (2010) and M.Sc. Degree (2013) in macromolecular science and engineering at Zhejiang University. His present scientific interests are focused on design, fabrication, and application of the smart interfacial materials and devices on the basis of bioinspired special wettability.

ABBREVIATIONS AFM, atomic force microscopy; ALD, atomic layer deposition; APPJ, atmospheric pressure plasma jet; APS, aminopropyltriethoxysilane; AVO, acid vapor oxidation; C12, dodecyltrichlorosilane; C16, hexadecyl trimethoxysilane; CNTs, carbon nanotubes; CVD, chemical vapor deposition; FAS, fluoroalkylsilane; FFM, friction force microscopic; FPU, triethoxysilyl-terminated fluoropolysiloxane; FRP, free-radical polymerization; FTIR, Fourier transform infrared spectroscopy; HDPE, high density polyethylene; HDTS, heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane; HRP, horseradish peroxide; LFS, liquid flame spray; NC, nitrocellulose; ODP, octadecylphosphonic acid; ODT, n-octadecanethiol; OPDA, octadecylphosphonic acid; OTHS, octadecyltrihydrosilane; OTS, octadecyltrimethoxysilane; PAA, poly(acrylic acid); PDAC, poly(diallyldimethylam10086


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monium chloride; PDDA, polydiallyldimethylammonium; PDMS, polydimethylsiloxane; PET, poly(ethylene terephthalate); PFAMA, perfluoroalkyl methacrylic copolymer; PFOA, perfluorooctanoic acid; PFTS, perfluorooctyltrichlorosilane; PMC, perfluoroalkylmethacrylic copolymer; PMMA, poly(methyl methacrylate); PMPS, polymethylphenyl-siloxane; POTS, perfluorooctyltriethoxysilane; PS, polystyrene; PSAA, poly(styrene-co-acrylic acid); P(S-BA-AA), poly(styrene-n-butyl acrylate-acrylic acid); PTFE, polytetrafluoroethylene; PVC, poly(vinyl chloride); PVDF, polyvinylidene fluoride; SA, selfassembly; SAM, self-assembled monolayers; SEM, scanning electron micrograph; TCPEOS, (3-chloropropyl) triethoxysilane; TEM, transmission electron microscopy; TMPSi, trimethoxypropyl silane; UV, ultraviolet; VOC, volatile organic compounds; VTEOS, vinyltriethoxylsilane; XPS, X-ray photoelectron spectroscopy

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