Self-cleaning materials have gained considerable attention for both their unique properties and practical applications in energy and environmental areas. Recent examples of many TiO2-derived materials have been illustrated to understand the fundamental principles of self-cleaning hydrophilic and hydrophobic surfaces. Various models including those proposed by Wenzel, Cassie-Baxter and Miwa-Hashimoto are discussed to explain the mechanism of self-cleaning. Examples of semiconductor surfaces exhibiting the simultaneous occurrence of superhydrophilic and superhydrophobic domains on the same surface are illustrated, which can have various advanced applications in microfluidics, printing, photovoltaic, biomedical devices, anti-bacterial surfaces and water purification. Several strategies to improve the efficiency of photocatalytic self-cleaning property have been discussed including doping with metals and non-metals, formation of hetero-junctions between TiO2 and other low bandgap semiconductors, and fabrication of graphene based semiconductor nano-composites. Different mechanisms such as band-gap narrowing, formation of localized energy levels within the bandgap and formation of intrinsic defects such as oxygen vacancies have been suggested to account for the improved activity of doped TiO2 photocatalysts. Various preparation routes for developing efficient superhydrophilic–superhydrophobic patterns have been reviewed. In addition, reversible photo-controlled surfaces with tuneable hydrophilic/hydrophobic properties and its technological applications are discussed. Examples of antireflective surfaces exhibiting self-cleaning properties for the applications in solar cells and flat panel displays have also been provided. Discussion is provided on TiO2 based self-cleaning materials exhibiting hydrophilic and underwater superoleophobic properties and their utilities in water management, antifouling applications and separation of oil in water emulsions are discussed. In addition, ISO testing methods (ISO 27448: 2009, ISO 10678: 2010 and ISO 27447: 2009) for analysing self-cleaning activity and antibacterial action have also been discussed. Rapid photocatalytic self-cleaning testing methods using various photocatalytic activity indicator inks such as resazurin (Rz), basic blue 66 (BB66) and acid violet 7(AV7) for a broad range of materials such as commercial paints, tiles and glasses are also described. Various commercial products such as glass, tiles, fabrics, cement and paint materials developed based on the principle of photo-induced hydrophilic conversion of TiO2 surfaces have also been provided. The wide ranges of practical applications of self-cleaning photocatalytic materials suggest further development to improve their efficiency and utilities. It was concluded that a rational fabrication of multifunctional photocatalytic materials by integrating biological inspired structures with tunable wettability would be favorable to address a number of existing environmental concerns.

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Self-Cleaning Applications of TiO2 by Photo-Induced Hydrophilicity and Photocatalysis

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

Bio-Inspired Titanium Dioxide Materials with Special Wettability and Their Applications

Silica-titania core-shell nanoparticles of 30, 40, 50, 55, 75, and 110 nm were prepared from tetraethyl orthosilicate (TEOS) and tetraisopropyl titanate (TIPT). After calcination, the amorphous titania shell transformed into anatase nanoparticles, and the silica-titania core-shell nanoparticles became raspberry-like nanoparticles. These nanoparticles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and UV-vis spectroscopy. Hierarchically structured antireflective and self-cleaning particulate coatings were fabricated on glass substrates via layer-by-layer (LbL) assembly using silica-titania core-shell nanoparticles and silica nanoparticles as building blocks followed by calcination. The maximum transmittance of coated glass substrates reached as high as ca. 97%, while that of the glass substrates is only ca. 91%. The morphologies of the coatings were observed by SEM and atom force microscopy (AFM). Such hierarchically structured raspberry-like SiO2-TiO2 nanoparticle coatings had superhydrophilic and antifogging properties. The coatings also showed photocatalytic activity toward organic pollutants and thus a self-cleaning property.

Synthesis of raspberry-like sio2–tio2 nanoparticles toward antireflective and self cleaning coatings

Current trends, challenges, and perspectives of anti-fogging technology: Surface and material design, fabrication strategies, and beyond

Transparent hydrophilic photocatalytic TiO2 coatings have been widely applied to endow the surfaces self-cleaning properties. A mixed metal oxide (TiO2/SiO2) can enhance the photocatalytic performance improving the ability of surface adsorption and increasing the amount of hydroxyl surface groups. The present work introduces a systematic study concerning the effect of the SiO2 addition to TiO2 films on the wettability, the photocatalytic activity, the adhesion strength, and the mechanical stability of the films. Transparent hydrophilic photocatalytic TiO2/SiO2 thin films were used to coat the polycarbonate (PC) substrate which was precoated by an intermediate SiO2 layer. The TiO2/SiO2 thin film was prepared employing a bulk TiO2 powder (Sachtleben Hombikat UV 100) and different molar ratios of tetraethoxysilane in acidic ethanol. A dip-coating process was used to deposit the films onto the polycarbonate substrate. The films were characterized by UV/vis spectrophotometry, FTIR spectroscopy, ellipsometry, B...

A. Bourgeois

Papers

Morphology and natural wettability properties of sol-gel derived TiO2-SiO2 composite thin films

Enhanced persistence of natural super-hydrophilicity in TiO2–SiO2 composite thin films deposited via a sol–gel route