Study of stimuli responsive behaviors and functional properties of TiO₂and other oxide-based nanomaterials

Abstract

This study is on the stimuli responsive behaviors and functional properties of titanium dioxide, TiO₂, one of the photosensitive materials that have aroused numerous research interests, in an attempt to create innovative and functional textiles. While the work started from and focused on nanoscaled TiO₂, it also extends to the other oxide-based nanomaterial, zinc oxide (ZnO). Inspired by the discovery of photo-induced superhydrophilic phenomenon of TiO₂ in 1997, this work firstly started with the fabrication of the smart fabric with one side having superhydrophilic and the other having hydrophobic properties. These two contrasting properties were obtained through the creation of surface energy gradient within 3-D fibrous matrix by taking advantage of the smart hydrophobic-hydrophilic properties associated with TiO₂ controlled by a "photo-and-dark" process. Combining the analysis of the results from scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Atomic Force Microscope (AFM), it was concluded that the rough nano-porous surface structure and hydrocarbon adsorption from air contribute to the hydrophobicity of the UV-opaque side of TiO₂-smart textile fabric. On UV-irradiated side of fabric, the UV-induced chemical heterogeneity of having both hydrophobic and hydrophilic domains in nanoscale formed within fibrous textile matrix contributes to the superhydrophilicity. With predetermined amount of UV irradiation, water can be transported in a controllable manner from the hydrophobic side to hydrophilic side and then spread out rapidly along the capillary channels on the hydrophilic side. The cooperation of 2D and 3D capillary effect created by nanoporous TiO₂ in fibrous matrix accounts for this directional wicking phenomenon. In a similar context, study on ZnO was also conducted. The simple method of creating a smart directional water transport textile described in this study is the first of its kind and it not only promotes the control of wetting behavior in a two-dimensional rigid surface toward three-dimensional flexible bulk matrix, but also opens up a new research direction for TiO₂ and other smart materials toward their widened applications in smart and intelligent systems. Secondly, to further explore the external stimuli in controlling wetting behavior of TiO₂, we investigated the dual-responsive TiO₂ surface wetting behavior controlled by "light-and-temperature". In comparison to the well known TiO₂ switchable wetting behavior controlled by "light-and-dark", it was found that TiO₂ surface can be reversibly altered from hydrophobic state to hydrophilic state in responsive to ultraviolet light and temperature. This research examines the temperature in a range from 2°C to 200°C for such process to find out its effect on the surface wettability of TiO₂. It is revealed that temperature plays an important role in controlling the speed of hydrophobic formation of TiO₂ thin film during the treatment process as well as hydrophobic re-formation after UV irradiation. High temperature enables a quick re-formation and low temperature slows it. To shed light on this interesting phenomenon, Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) was employed to explain the possible mechanism. AFM examines the nanoscaled surface structural change responsive to the two stimuli. XPS study reveals that the switchable surface wettability is related to the proportion of the hydrocarbons and hydroxyls presented on the surface. This work can lead to a deeper understanding on the study of stimuli responsive wetting behavior of TiO₂. Thirdly, the use of TiO₂ is extended to wool, one of the luxury fibers widely used in winter clothing. Wool requires special care and is not suitable for frequent washing. Photodegradation is another problem for wool. To promote the development of high performance and easy-care wool textiles, bi-functional wool fabrics with UV protection and photocatalytic self-cleaning properties were achieved by surface treatment using anatase nanotitania synthesized by a low temperature sol-gel process. The results show that nano-crystalline anatase TiO₂ thin film is formed on fiber surface as revealed by X-ray diffraction spectroscopy (XRD), field-emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX). TiO₂-treated wool performs photo-catalytic self-cleaning property as proved by the decomposition of methylene blue solution and good UV protection indicated by low UV transmittance.