Titanium Dioxide Derived Materials with Superwettability

: Titanium dioxide (TiO 2 ) is widely used in various ﬁelds both in daily life and industry owing to its excellent photoelectric properties and its induced superwettability. Over the past several decades, various methods have been reported to improve the wettability of TiO 2 and plenty of practical applications have been developed. The TiO 2 -derived materials with different morphologies display a variety of functions including photocatalysis, self-cleaning, oil-water separation, etc. Herein, various functions and applications of TiO 2 with superwettability are summarized and described in different sections. First, a brief introduction about the discovery of photoelectrodes made of TiO 2 is revealed. The ultra-fast spreading behaviors on TiO 2 are shown in the part of ultra-fast spreading with superwettability. The part of controllable wettability introduces the controllable wettability of TiO 2 -derived materials and their related applications. Recent developments of interfacial photocatalysis and photoelectrochemical reactions with TiO 2 are presented in the part of interfacial photocatalysis and photoelectrochemical reactions. The part of nanochannels for ion rectiﬁcation describes ion transportation in nanochannels based on TiO 2 -derived materials. In the ﬁnal section, a brief conclusion and a future outlook based on the superwettability of TiO 2 are shown.


Introduction
TiO 2 is widely used in practical applications and daily life due to its special properties [1], especially the applications of its photo-induced wettability. Many plants and animals in nature possess special wettability that can provide inspiration for research work, such as lotus leaves [2], fly eyes [3], gecko feet [4], the pitcher [5], etc. [6,7]. In 1964, Fowkes [8] provided a theory for calculating surface energy. He decomposed the surface tension into two forces: London dispersion force and polar force composed of dipole force, hydrogen bonds, and induced force. In 1969, Owens and Wendt [9] developed Fowkes theory, which considered that besides London dispersion force, there were polar effects including induced force, orientation force, and hydrogen bonds at the liquid-solid interface. In 1972, Fujishima and Honda [10] reported photolysis of water through an electrolytic cell that connects a TiO 2 electrode and a platinum electrode. When the TiO 2 electrode was irradiated by light, the current flow occurred and the oxygen was produced. The way oxygen is produced by oxidizing water is like the way it is produced by photosynthesis in nature. In 1997, Wang et al. [11] reported a property named "superamphiphilicity", which denotes that the static contact angles (CAs) of both water and oil are approaching 0 • on the surface of TiO 2 under UV illumination. The mechanism can be explained by the formation of oxygen vacancies and the conversion of Ti 4+ sites to Ti 3+ sites due to UV light. These defects influence the affinity to chemisorbed water at the surrounding sites formation of oxygen vacancies and the conversion of Ti 4+ sites to Ti 3+ sites due to UV light. These defects influence the affinity to chemisorbed water at the surrounding sites and, in turn, hydrophilic and hydrophobic nanodomains form on superamphiphilic TiO2 surfaces [12]. In addition, self-cleaning and anti-fogging characteristics are reported on TiO2 surfaces [11]. A growing number of researchers have paid attention to TiO2-derived materials with superwettability.

Ultra-Fast Spreading with Superwettability
Ultra-fast spreading has long been a research topic, and TiO 2 -based materials are one of the widely used superspreading materials due to their unique photocatalysis properties [7,38,39]. Ultra-fast spreading is a wetting phenomenon indicating liquid permeates and diffuses into complex surface structures containing pores, taking advantage of the capillary effect [40]. Liquid shows ultra-fast spreading behaviors on TiO 2 -derived materials because of the aforementioned photo-induced superwettability and the structure of the surface, which can be regulated on demand [3,[41][42][43].
The formation of nano-sized structures is helpful to the ultra-fast spreading behavior of droplets. Li et al. [44] reported nanostructured metal nitrides with the desirable property of superwettability. Compared to traditional TiN nanoparticle structures, the connected nanofibers and nanotubes structures have a larger surface-to-volume ratio, larger specific surface area, better mechanical stability, and independent transmission channels. A hierarchical amorphous TiO 2 nanoarray reported by Li et al. [45] showed superamphiphilicity without UV-illumination. This is due to the preparation method called pulsed laser deposition, which is more cost-effective than the traditional process. Meanwhile, many TiO 2 surfaces have special structures that show the property of superwettability. Zorba et al. [46] reported a self-similar porous TiO 2 surface with superhydrophilic properties without UVirradiation. The water CAs release to less than 5 • quickly, in 160 ms. With the introduction of TiO 2 onto the glass substrate, the coating showed obvious anti-fogging performance. Denison et al. [47] reported ultra-fast spreading behavior of water droplets on a transparent mesoporous superhydrophilic TiO 2 film prepared by reverse micelle, sol-gel, and spin coating technology. The photo-induced stick-slip behavior on mesoporous TiO 2 can be observed by investigating the spreading of water droplets on the semiconductor surface irradiated by ultraviolet light. By means of electrospinning, Ganesh et al. [48] reported that a rice-shaped TiO 2 nanostructure showed superhydrophilicity. The superhydrophilicity of TiO 2 coatings depends on their thickness. Electrospinning TiO 2 shows better photocatalytic performance than commercial Degussa P25. Funakoshi et al. [49] prepared a superhydrophilic anatase TiO 2 film by a titanium alkoxide hydrolysis operation. The water CAs and the transmittance of visible light to its substrate vary with the duration of tetraethyl orthotitanate (TEOT) hydrolysis operation. In the hydrolysis process of TEOT, the water CAs on the film shows about 35 • , and when the process is longer than 60 min, the CAs drops below 1 • . With the prolongation of the hydrolysis time of TEOT, the transmittance of light through its substrate is low.
In addition to the formation of nano-sized structures, fabrication of microstructures also influences surface superwettability. Kobayashi et al. [50] observed ultra-fast spreading on a TiO 2 micropillar arrays with overhanging roofs. When the geometrical angle between the overhanging roof and the vertical micropillar shaft decreased from 51 • to 9 • , the wettability transition time when storing in dark and under UV illumination decreased from 180 min to 6 min.
In contrast to nanostructures and microstructures, the micro-nano composite structure can make the surface ultra-fast spreading more obvious. Joung et al. [51] fabricated a superhydrophilic micro-nano composite porous TiO 2 structure by electrophoretic deposition and breakdown anodization. The thin film prepared by the electrophoretic deposition method shows a nanostructure and a basis for producing a superhydrophilic porous structure by the breakdown anodization method. On the basis of the superwettability property of TiO 2 , the spreading ability of TiO 2 can be greatly enhanced by adjusting the structure, such as a to a mesh structure. Wang et al. [52] reported a superhydrophilic TiO 2 fibrous mesh with an ultra-fast spreading property (Figure 2a). The mesh is made by tetrabutyl titanate and poly (vinyl pyrrolidone) composed of polycrystalline anatase TiO 2 , and it has three-dimensional micropores surrounded by nanofibers, and the nanochannel structure of a single fiber promotes the superhydrophilicity of water (Figure 2b). The water droplet spreads more quickly on the surface of a fiber mesh composed of the superhydrophilic fibers ( Figure 2c). The capillary effect in three dimensions induces the wetting of water in the vertical direction ( Figure 2d). The final wetting state shows that CAs is 0 • . Similar to TiO 2 fiber mesh, Wen et al. [32] reported superhydrophilic SiO 2 -TiO 2 porous nanofibrous membranes through electrospinning and calcination. This material shows the characteristics of ultra-fast water wettability ( Figure 2f) and the graded porous structure of the nanofiber membrane is beneficial for improving adsorption capacity and permeability. The membrane was useful for water purification (Figure 2g). Based on the micro-nano composite structure, the fabrication of the internal structure of the fiber depends on electrospray technology and helps to form a superhydrophilic surface. In 2008, Chen et al. [53] proposed a simple fluid electrospray technology to produce multi-component microcapsules. A novel multicompartment accomplished one-step multi-component encapsulation. This simple and effective composite fluid electrospray technique is crucial for separating and encapsulating active components. In 2010, Chen et al. [54] proposed a multifluidic coaxial electrospinning method for nanowire-in-microtube structured preparation. Compared with the multifluidic electrospinning method, the new one method adds an extra middle fluid between the polyacrylonitrile core and the polystyrene shell fibers. The advantage is the introduction of the additional intermediate fluid is that it can effectively reduce the interaction between the other two fluids. Further, in 2011, Chen et al. [55] reported a hierarchically porous inorganic nanofiber structure using microemulsion electrospinning. With the increase of precursor concentration, the diameter of nanopores increases. Meanwhile, the internal structure of the fiber can be controlled by changing the composition of micro-emulsion and the conditions of cations.

Controllable Wettability
In addition to superspreading properties, controllable superwettability, which denotes the effective conversion between superhydrophilicity and superhydrophobicity, is widely studied and used in daily life because of its applications in smart devices such as printing, microfluidics, and water collection [56][57][58][59][60].
UV-induced superwettability was first reported by Fujishima et al. [11]. With UV-light illuminated on TiO 2 film, the water CA is 0 • . After storing the film in the dark, a high CA of about 72 • of the film is recovered. After that, much related research was further carried out. Kang et al. [33] reported on the under-oil switchable wettability of TiO 2 nanotube arrays (TiO 2 -NTAs). Comparison between TiO 2 -NTAs and TiO 2 -films showed that they have the same superhydrophilicity in air but different wettability under-oil. With UV irradiation, TiO 2 -NTAs reveal CAs of about 0 • under oil. After heating at 150 • C in air, the superhydrophobicity appears again. Zeng et al. [61] reported a new heterostructure with the TiO 2 -NTAs composited with Ni(OH) 2 , which shows a photo-induced color change by oxidative energy storage. The oxidative energy is generated by TiO 2 nanotubular arrays under UV light, and the oxidative energy can be reserved in the Ni(OH) 2 layer. The color change phenomenon can be shown with reducing agents' bleaching such as with alcohol, hydrogen peroxide, and ketone solutions. This storage-releasing oxidative energy-induced color change switch shows good reversibility. Gao et al. [62] reported a switchable superwettability membrane called a biomimetic TiO 2 -titanium mesh. A controllable underwater superwettability was shown by UV-illumination and heating. In terms of nanotube array structure, Chagas et al. [63] reported fabrication of TiO 2 nanoparticle@trimethoxypropyl silane on a polypropylene substrate. The water CAs showed a change between superhydrophobic and superhydrophilic with UV light and heating. Additionally, UV light can increase the adhesion of the TiO 2 nanoparticle@trimethoxypropyl silane coating. Caputo et al. [64] reported controllable wettability on a colloidal TiO 2 nanocrystal film under UV irradiation. A low water CA was shown after UV illumination. Meanwhile, a vacuum environment can accelerate the change of wettability. Wang et al. [65] reported a nanotube-structured TiO 2 film with switchable water adhesion. The hydrophilic region is generated by ultraviolet irradiation, and the alternating hydrophilic/superhydrophobic alignment is realized by using a mask. This phenomenon was applied to potential in-water drop manipulation. Feng et al. [66] prepared a TiO 2 nanorod film with a special micro-nano composite structure on glass substrate, which shows superhydrophobic behavior. After UV irradiation, water CAs of the sample change from superhydrophobic to superhydrophilic. The TiO 2 nanorod is shown in Figure 3a,b. As a photosensitive material, with UV illumination, the photogenerated hole combines with lattice oxygen to form surface oxygen vacancies, then, the film revealed water CAs of about 0 • . After storage in the dark, finally, the surface wettability converted from superhydrophilic to superhydrophobic again (Figure 3c). Repeated reversible superhydrophilic-superhydrophobic transition is shown in Figure 3d.
After UV irradiation, water CAs of the sample change from superhydrophobic to superhydrophilic. The TiO2 nanorod is shown in Figure 3a,b. As a photosensitive material, with UV illumination, the photogenerated hole combines with lattice oxygen to form surface oxygen vacancies, then, the film revealed water CAs of about 0°. After storage in the dark, finally, the surface wettability converted from superhydrophilic to superhydrophobic again (Figure 3c). Repeated reversible superhydrophilic-superhydrophobic transition is shown in Figure 3d.  In addition to ultraviolet irradiation, other external stimuli can also be applied to achieve controllable wettability of TiO 2 . Lian et al. [67] reported a controllable wettability on TiO 2 rough surfaces with ethanol immersion and dark storage. The experiment phenomenon shows the changing CAs via the change of hydroxyl. A special nanostructure was reported by Lai et al. [68] who designed three controllable adhesion and superhydrophobic nanostructures, which include nanopores, nanotubes, and a nanovesuvianite array. The adhesion of water can be controlled by changing the diameter or length of nanotubes. This design guides the new functional nanomaterials with custom-tailored surface hydrophobicity and adhesion. Hu et al. [35] reported superhydrophobic TiO 2 -NTAs with controllable water adhesion. These TiO 2 -NTAs were modified by 1H,1H,2H,2Hperflfluorooctyl-triethoxysilane (PTES). The result shows that when a nanotube's diameter or length increased, the adhesion of water was reduced. Following this feature, a water droplet can be transferred from having low adhesive superhydrophobic TiO 2 -NTAs to high adhesive superhydrophobic TiO 2 -NTAs, and then to a smooth Ti film. The high adhesive TiO 2 -NTAs were used as "mechanical hands", and an anti-icing property was revealed on the low adhesion TiO 2 -NTA surface. Moreover, a switch of the electric field can control the dynamic wettability. Li et al. [69] reported a strategy for achieving controllable liquid manipulation and transportation on a micro-nano structured elastomer film via an electric field. Wettability is controlled according to the stretching/releasing of the elastomer TiO 2 surface. After coating with carbon grease and applying an electric field, when the voltage was applied, the shortening of the distance of the micro-nano composite structure clusters could be triggered. Liu et al. [70] reported a TiO 2 /reduced graphene oxide composite aerogel that was used for selectively degrading pollutants in water and depended on its photocatalysis properties. TiO 2 nanostructures have been grown on three-dimensional reduced graphene oxide porous structures and play an important role. Pan et al. [71] reported a TiO 2 nanoparticle (NP) coating modified with 1H,1H,2H,2Hperfluorooctyl(trimethoxy)silane. The CAs could rapidly change from 165 • to 0 • in 10 min after heating. Meanwhile, the membrane can be used to separate oil and water.
Controllable wettability can be applied to oil-water separation. Shi et al. [72] fabricated a TiO 2 @copper wire mesh. The TiO 2 @copper wire mesh covered with a TiO 2 micro-nano composite structure has a transforming wettability of less than 30 min in addition to its property of switchable wettability applied for oil-water separation. Yan et al. [73] designed a carbon nanotubes/TiO 2 composite membrane, which was used to separate oil-water systems. The membrane exhibited high separation efficiency in oil-water separation. With the corrosive test and after several cycles, the material also showed a stable separation effect. Ren et al. [74] reported a superhydrophobic TiO 2 @CA/AS coating that was fabricated with chitosan (CS) and stearic acid (SA). The wettability can be controlled with ammonic and heat treatment and is used for separating oil and water.
Moreover, patterned printing based on controllable wettability has also become attractive. Tian et al. [34] reported a superhydrophobic nanorod array surface whose patterned wettability transition could be controlled by the photoelectric cooperative. Under the condition of only applying voltage, the wettability of the nanorod surface cannot be regulated. However, under a certain voltage, by controlling illumination, a controllable wettability interface with photoelectric cooperation can be formed (Figure 4a,b). With this property, a desired pattern "H" is obtained (Figure 4c). Guo et al. [75] reported an approach for patterned liquid permeation with a TiO 2 -NTAs coated Ti mesh. The wettability of this coated material can be controlled by the photoelectric response. The mechanism can be explained as follows. At first, the surface of the superhydrophobic Ti mesh with TiO 2 -NTAs becomes hydrophilic by introducing appropriate voltage and light. However, if the voltage is too high or too low, the hydrophilic phenomenon of photoelectric synergy cannot be obtained. In the same year, Guo et al. [76] reported a ZnO mesh surface with the micro-nano composite structure for liquid patterned printing. By adding voltage and illumination, a great difference can be created between patterned and un-patterned areas due to the micro-nano composite structure of the ZnO mesh. Patterned liquid printing is realized in this way. With different mechanisms on patterned printing, Zheng et al. [57] reported a TiO 2 -SiO 2 composite coating used for electrostatic patterning. Temporary conductivity of the insulation surface was realized by electrostatic powder spraying. The water droplets sprayed on the superhydrophilic surface can diffuse rapidly and spread into a water film, and the first pass transfer efficiency is promoted to more than 80%. After spraying the powder particles on the water film and heating it, an epoxy-polyester film on a porous composite coating was accomplished (Figure 4d,e). Surface patterning was completed by electrostatic attraction of negative charge of epoxy polyester powder and positive charge of water (Figure 4f,g). The micro-grade lines with different sizes are shown in Figure 4h. ter droplets sprayed on the superhydrophilic surface can diffuse rapidly and spread into a water film, and the first pass transfer efficiency is promoted to more than 80%. After spraying the powder particles on the water film and heating it, an epoxy-polyester film on a porous composite coating was accomplished (Figure 4d,e). Surface patterning was completed by electrostatic attraction of negative charge of epoxy polyester powder and positive charge of water (Figure 4f,g). The micro-grade lines with different sizes are shown in Figure 4h.

Interfacial Photocatalysis and Photoelectrochemical Reactions
Due to the photogenerated electron effect of TiO2, more and more practical applications are emerging in interfacial photocatalysis and photoelectrochemical (PEC) reactions. For interfacial photocatalysis, a photodegradation system composed of TiO2 has an advantage in environmental protection and renewable energy, which has attracted the attention of researchers [77,78].
Niu et al. [79] reported a TiO2/polypropylene composite membrane on which the flowing organic dyes can be completely photodegraded under the irradiation of ultraviolet rays. During the process, the aqueous solution selectively wetted the hydrophilic porous nano-TiO2 and skipped the superhydrophobic substrate, meaning most of the dye molecules were in direct contact with the active site of TiO2 for complete degradation. The

Interfacial Photocatalysis and Photoelectrochemical Reactions
Due to the photogenerated electron effect of TiO 2 , more and more practical applications are emerging in interfacial photocatalysis and photoelectrochemical (PEC) reactions. For interfacial photocatalysis, a photodegradation system composed of TiO 2 has an advantage in environmental protection and renewable energy, which has attracted the attention of researchers [77,78].
Niu et al. [79] reported a TiO 2 /polypropylene composite membrane on which the flowing organic dyes can be completely photodegraded under the irradiation of ultraviolet rays. During the process, the aqueous solution selectively wetted the hydrophilic porous nano-TiO 2 and skipped the superhydrophobic substrate, meaning most of the dye molecules were in direct contact with the active site of TiO 2 for complete degradation. The bleaching effect of congo red dye was better with the increase of TiO 2 NP. Tao et al. [80] reported a nano-porous anatase TiO 2 film deposited with Cu x O quantum dots through a continuous ionic layer adsorption reaction. The microstructure, morphology, and loading capacity of Cu x O quantum dots on TiO 2 thin films were controlled by changing the cycle times of successive ionic layer adsorption reactions. This kind of electrode showed better photoelectric performance and might be applied to solar energy conversion and water degradation. These applications often rely on rapid wetting where the superwettability of TiO 2 can be promoted.
For PEC reactions, TiO 2 can participate in the formation of bioelectrodes. Recent works have reported on bioelectrodes and triphase reactions. The solid-liquid-gas triphase reaction, which is based on the bioelectrode, exhibited higher detection sensitivity and a larger detection range of the sensor than those of the normal solid-liquid diphase reaction. It is precisely because of the superwettability of TiO 2 that the triphase reaction system can be formed. These works enlighten our further understanding and research in this field [81][82][83]. Photoelectrochemical reactions applied to PEC biological detection systems have been studied. Wang et al. [36] reported a PEC biological detection system. The SEM image showed that TiO 2 nanowires (NWs) were fabricated on transparent fluorine-doped tin oxide-coated glass (Figure 5a) and their surfaces were completely covered by a graphene oxide/chitosan composite layer (Figure 5b). This system was composed of a photoanode composed of single crystal semiconductor NWs and a biocathode stationary oxidase on the surface of the superhydrophobic NWs (Figure 5c). H 2 O 2 played a crucial role in the reductase reaction occurring at the three-phase contact interface (Figure 5d,e). This kind of PEC bioassay system had the advantage of higher detection sensitivity and range. Compared with the diphase PEC system in a PBS solution at different oxygen concentrations, photocurrents measured with different H 2 O 2 concentrations showed that the triphase bioassay system revealed stable electrolyte oxygen levels, which means the fluctuation was small (Figure 5f). The strong photocurrent response appeared at 0 V, which means that the three-phase measurement process can be carried out without external bias (Figure 5g). With a similar application, Chen et al. [84] designed a triphase bio-photoelectrode that was built with superhydrophobic TiO 2 NW surfaces and oxidase was fixed to the nanowire array. The bio-photoelectrode can obviously enhance the oxidase kinetics, efficient transfer, and collection of photogenerated charge. The three-phase bio-photochemical detection system formed by the electrode promoted beneficial effects in medical treatment and clinical diagnosis, such as blood glucose, lactic acid, and cholesterol detection. In terms of modified materials, Zhang et al. [85] reported a method to control the PEC water-splitting performance on the TiO 2 -NTA surface. The result showed water CAs of about 134 • , and octadecyltrimethoxysilane (OTS)-modified TiO 2 nanotubes revealed better PEC properties. This was because of the suppression of the recombination of photogenerated electrons and holes and the improvement of the injection efficiency of photogenerated holes. However, its excessive thickness will affect the light absorption and inhibit the PEC properties.
Photocatalysis and PEC reactions can be applied in biomimetic materials and the osseointegration field. Rafieerad et al. [86] reported mixed oxide nanotubes (titania-niobiaalumina) that were prepared on an alloy, called Ti6Al7Nb implants, by physical vapor deposition (PVD) magnetron sputtering and the electrochemical method. First, under the optimized coating conditions, a thin film of niobium (Nb) with good adhesion was sputtered by PVD, and then nanotube arrays were grown on the surface of Nb/Ti6Al7Nb implants after anodizing and heat treatment. Last, the prepared graphene oxide (GO) nanosheets were loaded on the anode nanotubes to enhance the ternary ceramic membrane. Without GO, the water CA of mixed oxide nanotubes is 34.2 • , which drops sharply to 23.6 • after GO loading. The results showed that compared with mixed oxide nanotubes, the hybrid composite of NTAs and GO provided a better connection between implant and bone. Catalysts 2021, 11, x FOR PEER REVIEW 10 of 17

Nanochannels for Ion Rectification
In addition to controlling the wetting behavior of water molecules with superwettability, TiO 2 can also regulate the transport of ions in nanochannels. Size effect and surface charges are critical factors influencing the property of ion transport when nanochannels are constructed. Various significant properties can be found on TiO 2 -derived materials [87]. Inspired by biological ion channels, artificial nanochannels were prepared [88]. Plenty of research has been devoted to exploring the fundamental mechanism of nanochannels in recent years [89][90][91][92]. For bio-inspired ionic nano-channels, the importance of nano-scale ion transport based on TiO 2 -derived materials will be further enlarged in future biology and medicine [93][94][95][96][97].
The application of heterogeneous nanochannels for ion rectification was confirmed based on TiO 2 . Zhang et al. [98] fabricated TiO 2 -NTAs that benefitted ion channel transport. The artificial TiO 2 -NTAs with asymmetric structures can be used as ion channels regulated by UV light. UV light induced superwettability and inhibited ion transmission under positive voltage, while it promoted ion transmission under negative voltage. Liu et al. [99] reported TiO 2 microspheres with mesoporous channels that were grown in situ at the tips of glass microtubes by confined space evaporation. The TiO 2 micro-plug with asymmetric mesoporous channel showed strong ion current rectification abilities even in a saturated KCl solution.
With the modification of TiO 2 , Zhang et al. [37] reported a heterogeneous nanochannel composed of alumina and anatase type TiO 2 with OTS molecules (Figure 6a-e). The material under UV light showed the characteristics of ion gating and ion rectification (Figure 6f,g). This can be explained by UV light-induced asymmetric distribution of negative surface charges, which form modified organic molecules on the surface of TiO 2 . The artificial TiO 2 nanocomposites with asymmetric structures can be used as ion channels regulated by UV light, which affects not only surface wettability, controlling solution transmittance, but also decomposition of modified organic molecules on TiO 2 . To be specific, TiO 2 can transform into a superhydrophilic state under UV irradiation. Meanwhile, UV irradiation induced the occurrence of hole-electron pairs in TiO 2 and thus an oxidation of the organic molecules on surface, similarly to OTS. Decomposed organic anions form an electric field, which inhibist ion transmission under positive voltage (Figure 6h-k) [100]. It can be used in sensors and separation technologies. Hu et al. [101] described OTS modified TiO 2 -NTAs used for ion rectification. OTS was decomposed by UV illumination and carboxyl was introduced to the tip of the nanochannels. The water CAs of the tip side of the material proved this point. This artificial large-size nanochannel can be used for nano-fluidic diodes, sensors, and separations. The experiment revealed the intensity and direction of ion current rectification could be adjusted by pH or through the modification of citric acid. On the basis of the heterogeneous nanochannels design, ion rectification can be enhanced by loading Au nanoparticles. Yang et al. [102] reported TiO 2 nanochannel membranes loaded with spherical Au nanoparticles for near-infrared-gated artificial ionic nanochannels by a self-organizing anodization method. Owing to the limited light penetration depth, Au nanoparticles were deposited on the small tip side of the TiO 2 nanochannel membranes. This asymmetric design showed obvious photothermal conversion. Through carbon coating, Wang et al. [103] fabricated a nanopore structure that realized high-efficiency ion accessibility and fast ion transmission. This TiO 2 -C hybrid nanopore structure was synthesized by electrospinning and calcination and used as the electrode. The electrode showed high ion accessibility. After about 100 cycles, its capacity increased slightly, completing the process of "self-improvement". penetration depth, Au nanoparticles were deposited on the small tip side of the TiO2 nanochannel membranes. This asymmetric design showed obvious photothermal conversion. Through carbon coating, Wang et al. [103] fabricated a nanopore structure that realized high-efficiency ion accessibility and fast ion transmission. This TiO2-C hybrid nanopore structure was synthesized by electrospinning and calcination and used as the electrode. The electrode showed high ion accessibility. After about 100 cycles, its capacity increased slightly, completing the process of "self-improvement".

Conclusions
TiO2-derived materials with superwettability reveal a great deal of significant properties such as being amphoteric oxides. We described some of the major works, which include those on ultra-fast spreading, controllable wettability, interfacial photocatalysis, and nanochannels for ion rectification. As a material with superior performance in selfcleaning and photocatalysis, the TiO2-based material with its superwettability should be studied further, especially in terms of mechanisms for the ultra-fast spreading of TiO2, and more work should focus on the interfacial triphase reaction.
Nature is the inspiration source of biomimetic materials. Interfacial catalysis and interfacial reactions benefit from nature. On this basis, the research of multi-functional TiO2 materials is an inevitable trend. How to combine superwettability with other functions will be the emphasis of future work. In recent years, the environmental pollution and energy problems have become more serious. We believe that the study of TiO2 multi-functional materials will provide researchers with a favorable solution. (e) After UV irradiation, part of the TiO 2 nanochannels became hydrophilic. (f,g) Image of the I-V property of TiO 2 /Al 2 O 3 nanochannels by OTS modified/unmodified UV irradiation on TiO 2 . Reprint from [37]. (h) Fewer decomposed organic anions formed and smaller ion currents appeared without UV irradiation. (i) Some decomposed organic anions formed and normal ion currents appeared with short-term UV irradiation. (j) More decomposed organic anions formed and larger ion currents sappeared with long-term UV irradiation. Copyright (2014), with permission from WILEY-VCH, Weinheim, Germany. (k) Schematic diagram of photoexcited electron transition. Reprint from [100]. Copyright (1995), with permission from American Chemical Society, Washington, DC, USA.

Conclusions
TiO 2 -derived materials with superwettability reveal a great deal of significant properties such as being amphoteric oxides. We described some of the major works, which include those on ultra-fast spreading, controllable wettability, interfacial photocatalysis, and nanochannels for ion rectification. As a material with superior performance in selfcleaning and photocatalysis, the TiO 2 -based material with its superwettability should be studied further, especially in terms of mechanisms for the ultra-fast spreading of TiO 2 , and more work should focus on the interfacial triphase reaction.
Nature is the inspiration source of biomimetic materials. Interfacial catalysis and interfacial reactions benefit from nature. On this basis, the research of multi-functional TiO 2 materials is an inevitable trend. How to combine superwettability with other functions will be the emphasis of future work. In recent years, the environmental pollution and energy problems have become more serious. We believe that the study of TiO 2 multi-functional materials will provide researchers with a favorable solution.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.