Controlling Hydrophilicity of Titanium Dioxide for Self-cleaning Surfaces

Introduction Titanium dioxide (TiO2) exhibits interesting properties when exposed to light. Since the 1960's, light-induced properties of TiO2 were discovered and utilized such as electrochemical... [ view full abstract ]

• Introduction

Titanium dioxide (TiO₂) exhibits interesting properties when exposed to light. Since the 1960's, light-induced properties of TiO₂ were discovered and utilized such as electrochemical properties, photocatalytic purification, and hydrophilic properties. The prominence of TiO₂ as a hydrophilic coating is well established in different industries of which are fabrics, construction, windows, and solar panels. In this work, we apply a physical technique of nanofabrication that allows the controlling of the hydrophilicity of TiO₂. The TiO₂ thin-films were deposited at different thicknesses using electron-beam physical vapor deposition (e-beam PVD) technique on quartz substrates. Surface topography of the TiO₂ thin-films were studied using atomic force microscopy (AFM) and investigation of hydrophilicity was conducted by water contact angle measurements. We have noticed a remarkable increase in surface hydrophilicity after applying thermal annealing on the as-deposited TiO₂ thin-film samples; with contact angle (CA) dropping from around 69^o for as-deposited thin-films down to 20.2^o for annealed thin-films.

• Methods

Thin-films of TiO₂ were deposited using e-beam PVD in an ultra-high vacuum chamber (10^-6torr) at thicknesses varying between 5nm and 60nm. The as-deposited samples were then thermally annealed to form nanostructures - in a thermal tube furnace at 800^oC and under inert atmospheric pressure. The samples were characterized by AFM in contact mode to study surface topography of as-deposited thin-films and the annealed as well. In addition, deionized-water contact angle measurements were performed to evaluate surface hydrophilicity.

• Results And Discussions

Figure 1 presents 2D and 3D topography images of the deposited 60nm TiO₂ thin-film. Figure 1 (a) and (b) show as-deposited, while Figure 1 (c) and (d) show the annealed samples; showing a slight change in the surface structure. The roughness value (RMS) decreased from (3.38 to 2.82 nm). The contact angle measurements illustrated in Figure 2 indicate a drastic decrease in average contact angle from 69.2^o to 20.2^o. Suggesting that the formation of TiO₂ nanostructures by thermal annealing of as-deposited TiO₂ thin-films result in a better hydrophilic surface. The other studied thicknesses followed a similar trend; and a thickness-dependent hydrophilicity behavior was noticed, allowing for the hydrophilicity of TiO₂ coated surfaces to be employed as desired.