Hydrophilic Titania Thin Films from a Molecular Precursor Film Formed via Electrospray Deposition on a Quartz Glass Substrate Precoated with Carbon Nanotubes

Hiroki Nagai,Ryuhei Ozawa,Mitsunobu Sato,Natangue Heita Shafudah,Takeyoshi Onuma,Yukihiro Kudoh,Yutaka Suwazono,Taiju Takahashi

doi:10.3390/coatings10111050

Abstract

Titania precursor films were electrosprayed on a quartz glass substrate, which was pre-modified with an ultra-thin film obtained by spin-coating a single-walled carbon nanotube (SWCNT) dispersed solution. The X-ray diffraction patterns of the thin films obtained by heat-treating the precursor films at 500 °C in air for 1 h indicated that the formed crystals were anatase. A new route to fabricate transparent thin films on the insulating substrate via electrospray deposition (ESD) was thus attained. The photoluminescence spectrum of the thin film showed a peak at 2.23 eV, assignable to the self-trapped exciton of anatase. The Raman spectrum of the thin film demonstrated that heat treatment is useful for removing SWCNTs. The thin film showed a water contact angle of 14 ± 2° even after being kept under dark conditions for 1 h, indicating a high level of hydrophilicity. Additionally, the thin film had a super-hydrophilic surface with a water contact angle of 1 ± 1° after ultraviolet light irradiation with an intensity of 4.5 mW cm−2 at 365 nm for 1 h. The importance of Ti3+ ions in the co-present amorphous phase, which was dominantly formed via the ESD process, for hydrophilicity was also clarified by means of X-ray photoelectron spectroscopy.

Highlights

Metal oxide thin films have been widely applied to self-cleaning and anti-fogging of superhydrophilic glasses and mirrors [1,2]
The heat treatment of the precursor film at 500 ◦ C in air was useful for fabricating a titania thin film with a smooth surface and strong adhesion on the substrate, as well as to remove the precoated single-walled carbon nanotube (SWCNT) ultra-thin film
The relative amounts of crystallized anatase after the precursor films, which are heat-treated at 500 ◦ C for 1 h, are the major differences shown by their X-ray diffraction (XRD), Raman, and photoluminescence spectra

Summary

Introduction

Metal oxide thin films have been widely applied to self-cleaning and anti-fogging of superhydrophilic glasses and mirrors [1,2]. One approach involves irradiation of photoreactive materials such as titania (TiO2 ) [3], zinc oxide (ZnO) [4], and zirconia (ZrO2 ) [5] (photoinduced superhydrophilicity). This phenomenon is caused by light irradiation and loses superhydrophilicity when placed in the dark, resulting in a high-water contact angle of over 50◦. Another approach involves the use of a porous surface having a surface texture that promotes hydrophilic behavior [6]. The adhesion strength of the porous thin films to the substrate is generally low

Methods

Discussion

Conclusion