*3.2.1 Superhydrophilic photocatalysts*

Inspired by the natural world's self-cleaning as well as the water-repellent properties of the lotus leaf, superwetting materials with unique wettability are believed to

#### **Figure 4.**

*Schematic representation of a triphasic nanoarray photocatalyst and the photocatalytic water purification process.*

be promising materials for removing organic pollutants from water. Historically, the study of the superhydrophilicity of titanium dioxide (TiO2) films traces back to 1997. Before illumination by UV light, the contact angle of TiO2 surface was 72°. But after the UV illumination on the particular duration, the droplets completely spread on. This is due to creating numerous high-energy domains with hydrophilic/oleophilic properties on TiO2 surfaces. Moreover, the wetting properties of single TiO2 surfaces could be exchange between hydrophobicity and superhydrophilicity under the interchange of long-term dark storage and UV light irradiation.

After discovering its confirmed that, the TiO2 surface with superamphiphilic ability has unique wetting transition under UV light [12, 13]. Furthermore, Wang and co-workers reported a hydrophilic TiO2-coated glass with effective photogeneration, displaying antifogging and self-cleaning induced by UV illumination [14]. Then, Fujishima et al. confirmed the nanostructure TiO2/SiO2 films shows superwettability under UV irradiation. In the case, the upper part of TiO2 layer and the bottom part of porous SiO2 layer with a low refractive index providing platforms for self-cleaning and antifogging/reflection [15]. Also, Shang et al. fabricated visible active N-F doped TiO2 Nanotube and palladium oxide is decorated on the surface of the nano array [16]. Due to their superior photocatalytic property and particular nanoarray alignment, it gives promising self-cleaning applications. Since then, the superwetting approach has been frequently used for antifogging and self-cleaning applications. Jiang's group fabricated translucent and stable Ag@AgCl/g-C3N4/TiO2 ceramic films that showed superhydrophilicity and excellent photocatalytic activities for Rhodamine B degradation under visible and complete spectral irradiations. In this case, the water molecules

### *A Triphasic Superwetting Catalyst for Photocatalytic Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.109509*

from air can occupy the oxygen vacancies of TiO2 of the composite and produce hydroxyl groups, which makes the TiO2 more hydrophilic. So, by mixing P25 with g-C3N4 in a colloidal silica system, nano TiO2 particles can be dispersed and attached to the g-C3N4 particles, leading to increased surface roughness and hydrophilicity of the film systems. In this case, the hydroxyl groups of P25 may interconnect with that of silica particles which helps to increase the bonding strength of graphitic carbon nitride composite film and silicate glass. Besides adding P25 into the film system, the catalytic efficiency is improved [17]. Then, the Zhang group studied the Polymerbased nanocomposites functionalization by organic moieties to make superhydrophilicity. Afterwards, TiO2 nanoparticles coated with hydroxyethyl acrylate (HEA) without any solvent formed high durable superhydrophilic catalyst [18]. Compared with the bare TiO2 films, the TiO2 nanotube array film has excellent photocatalytic efficiency in terms of methyl orange (MO) degradation is reported [19].

Chen and co-workers reported hydrophilic interface engineering of the hydrophilic CoOx modified hydrophobic Ta3N5, which improves its water oxidation efficiency under visible light irradiation. Compared to the pristine Ta3N5 surface, CoOx deposited onto the MgO–Ta3N5 surface showed a 23-fold improvement [20]. Similarly, core-shell NaYF4:Yb, Tm@TiO2 NPS is fabricated for photocatalytic activities. Here, the hydrophilic layers of TiO2 were coated onto hydrophobic NaYF4:Yb materials and the Tm nanoplates are partially exchanging with oleic acid ligands which shows hydrophobic in nature into cetyltrimethylammonium bromide (CTAB) surfactants which is amphiphilic character. The combination of NaYF4:Yb, Tm (up conversion materials) with TiO2 (wide bandgap) with broad spectrum absorption changes the wettability of a solid surface to achieve high-quality interfaces in photocatalysts for smooth carrier migration [21].

### *3.2.2 Superhydrophobic photocatalysts*

Superhydrophobic metal oxide like ZnO [22], exhibits advanced photocatalytic activity; however, during prolonged UV irradiation, superhydrophobicity changes into superhydrophilicity, resulting from the easy decompositions of low-surface-energy compositions under the stimulus of light. Consequently, a photocatalyst showing long-term superhydrophobicity was once considered not to exist. More generally, it is undoubtedly necessary to modify the surface of the catalyst with stable hydrophobic organics, which are chemically and directly bonded. Therefore, to achieve photocatalytically active hydrophobic materials, researchers have combined metal-oxide particles with hydrophobic polymers like Polydimethylsiloxane (PDMS) and polytetrafluoroethylene (PTFE) as composite mixtures [23–25]. Recently, metal oxides (TiO2) and nonwetting organic polymers, namely epoxy resin, followed by grafting 1H,1H,2H,2Hperfluorooctyltriethoxysilan (PFOS), we prepared an inorganic-organic superhydrophobic paint (IOS-PA) used for photocatalytic removal of three organic dyes, Nile red, methyl blue, and methyl orange [26]. PDMS, PTFE, and silicone nanofilaments have also been used to conduct long-term superhydrophobicity and photocatalysis on one surface [26–29]. Sheng and co-workers established a novel triphase photocatalytic system by creating a unique photocatalyst in which TiO2 nanoparticles (NPs) were immobilized on carbon fiber (CF) substrate treated by poly(tetrafluoroethylene) (PTFE) for water pollution remediation [30]. After immobilizing TiO2, the surface of the materials are changed from superhydrophobic to hydrophilic. In the time of photocatalytic reactions, the TiO2 will be hydrophilic part while the substrate will be the superhydrophobic state which was connected with the atmosphere. Hence, the total system should

have an abundant triphasic contact area, which allowed a sufficient oxygen transport and the rapid generation of reactive oxygen species for organic pollutants degradation. Recently, a superhydrophobic (SHB) TiO2 nanoarrays catalyst with low surface energy and rough surface microstructure was reported as a model photocatalyst. The soft surface energy and rough surface microstructures of the SHB nanoarrays give the photocatalytic system long-range hydrophobic in nature and helps to introduce the triphasic reaction interface [31]. Superhydrophobicity is an integral part of self-cleaning on a photocatalyst which showed the synergistic effect of strong water repellency and photocatalytic activity [28], where the rolling drops remove macroscopic particles and the photocatalytic degradation ensure by UV or solar light. In a superhydrophobic system's air-water-solid triphase joint interface [27, 32, 33], a continuous and steady gas channel is recognized, providing abundant gaseous reactants and the resulting quick gas transportations. This system overcomes the drawbacks of weak dissolved gas transfer and low solubility in liquid-solid diphase reaction systems. Thus, photocatalytic activity efficiency and selectivity are sharply increased. Jinxiu groups reported about the oil–water mixture separation and photocatalytic degradation of quinoline blue, rhodamine B, methyl orange and methylene blue by using [Ni(DMG)2] hollow microtubes. The prepared [Ni(DMG)2] films is act as superhydrophobicity and superoleophilicity and ascribed to the Cassie–Baxter model. Similarly, Ag/TiO2@PDMS coated cotton fabric which is low-cost effective, and recyclable separation material used for water purification to degrade methylene blue (MB) [34]. The effect of a grafted PDMS layer on wetting properties of TiO2 for photocatalytical application is studied by Butt group [35]. The most effective dual-purpose ceria nanoparticle membrane is fabricated by facile spray-deposition method on stainless steel membrane for oil-water separation and photocatalytic degradation. The prepared membrane has superwetting properties which is efficient for oil/water separation. In this case the oil is passing through the stainless-steel membrane, whereas, high column of water is blocked. Furthermore, the CeO2 coated membrane is utilized for the efficient degradation of a dye [36].
