Adsorption-Controlled Wettability and Self-Cleaning of TiO₂

Abstract

Molecular adsorption on solids is inevitable and has significant influences on the wettability of materials, while the tuning mechanism of the wettability from molecular adsorption is yet to be understood. Using molecular dynamics (MD) simulations, we comprehensively studied the relation between the wettability of the TiO 2 surface and the adsorption of water and carboxylic acid molecules. Our results reveal that the increasing amount of surface hydroxyl groups from the decomposition adsorption of H 2 O increases the hydrophilicity of TiO 2 , providing molecular-level evidence for the previously proposed mechanism of photo-induced hydrophilicity. By contrast, the surface wettability becomes tunable with water contact angles changing from 0 to ∼130°through length adjustment of the adsorbed carboxylic acids. The TiO 2 surface is hydrophilic with the adsorption of shortalkyl-chain carboxylic acids (e.g., HCOOH) and becomes hydrophobic when longer-alkyl-chain carboxylic acids (H(CH 2 ) n COOH, n > 2) are present. Furthermore, long-alkyl-chain acids also increase surface oleophilicity, while the adsorption of HCOOH and CH 3 COOH significantly enhances the oleophobicity of TiO 2 . Water molecules can also more easily penetrate the space between oily contaminants and adsorbed short acid molecules, thereby further increasing its self-cleaning capacity. The present simulations not only reveal the mechanism of wettability caused by molecular adsorption but also provide a promising method to create materials with controllable wettability and high self-cleaning efficiency.

Effect of the Photoreduction Process on the Self-Cleaning and Antibacterial Activity of Au-Doped TiO2 Colloids on Cotton Fabric

“…For the latter mechanism, a thin layer of photocatalytic nanomaterials such as titanium dioxide (TiO 2 ), zinc oxide (ZnO), and graphitic carbon nitride nanosheets (g-C 3 N 4 ) is applied to a textile substrate and then immobilized via different techniques . The photocatalytic nature of the nanocoatings breaks down the adsorbed organic contaminants such as stains and microorganisms into harmless products under light irradiation . For this type of nanocoating, hydrophilicity is an advantage as it can boost the photocatalytic activity, which results in an enhanced self-cleaning property. , Among different types of photocatalysts, TiO 2 has been widely used in the surface functionalization of textiles through different methods such as sol–gel, sonication, hydrothermal, and cross-linking on various types of fabrics including cotton, wool, polyester, cashmere, and silk, among others .…”

Photoelectrochemical Methods for the Determination of the Flat-Band Potential in Semiconducting Photocatalysts: A Comparison Study

“…Half a century has passed since Fujishima and Honda discovered water splitting in a photoelectrochemical cell with the photoanode made from TiO 2 . This distinguished work stimulated great research interest in photocatalysis for energy and environmental applications. − Many semiconductors have been studied in the photocatalytic and photoelectrocatalytic processes of H 2 evolution, CO 2 reduction, and pollutant degradation. − Semiconducting binary and ternary oxides (e.g., TiO 2 , WO 3 , Bi 2 WO 6 , and Bi 2 MoO 6 ) are commonly used for oxidative photocatalytic degradation of organic micropollutants on surfaces or in water and air. , Another striking example is the cobalt oxide (CoO) photocatalyst, which exhibits high efficiency in photocatalytic overall water splitting reactions. , Metal sulfide and metal selenide photocatalysts (e.g., CdS, ZnIn 2 S 4 , CdSe, and ZnSe) have also been studied intensively. , Among polymeric materials, graphitic carbon nitride (g-C 3 N 4 ) is the most popular and promising photocatalyst due to simple preparation techniques and great activity in photocatalytic reactions of hydrogen evolution and CO 2 photoreduction. , Currently, one of the most efficient strategies for the development of novel photocatalysts with enhanced photocatalytic performance is the design of nanostructured composites using several semiconductors to have the possibility of heterojunction of photogenerated charge carriers. , …”