Light-Controlled ZrO2 Surface Hydrophilicity

Abstract

In recent years many works are aimed at finding a method of controllable switching between hydrophilicity and hydrophobicity of a surface. The hydrophilic surface state is generally determined by its energy. Change in the surface energy can be realized in several different ways. Here we report the ability to control the surface wettability of zirconium dioxide nano-coatings by changing the composition of actinic light. Such unique photoinduced hydrophilic behavior of ZrO₂ surface is ascribed to the formation of different active surface states under photoexcitation in intrinsic and extrinsic ZrO₂ absorption regions. The sequential effect of different actinic lights on the surface hydrophilicity of zirconia is found to be repeatable and reversibly switchable from a highly hydrophilic state to a more hydrophobic state. The observed light-controllable reversible and reproducible switching of hydrophilicity opens new possible ways for the application of ZrO₂ based materials.

Figure 1. Light-induced hydrophilic behavior of ZrO₂ films.

(a) Photo images of water droplets on the ZrO₂ surface. (b) Optical images of water droplets on the ZrO₂ surface. 1, An initial hydrophilic state before irradiation. 2, A superhydrophilic surface after UV-irradiation, λ < 300 nm. 3, A hydrophobic surface after irradiation by light with λ > 300 nm. Θ is the water contact angle.

Figure 2. Reversible switching between superhydrophilic and less hydrophilic states of ZrO₂ by alteration of the spectral composition of actinic light: hv₁ corresponds to light with λ < 300 nm, hv₂ – to light with λ > 300 nm.

Figure 3

(a) Excitation spectrum of ZrO₂ photoluminescence with emission at 430 nm (2.89 eV). (b) Emission spectrum of the 30‒W deuterium lamp DDS-30 (LOMO). (c) Emission spectrum of the high-pressure 120-W mercury vapour lamp (LOMO). Emission spectra of the lamps were normalized to an irradiance of 0.1 mWm⁻² nm⁻¹ and 2.4 mWm⁻² nm⁻¹ for deuterium and mercury lamps, respectively. The dashed line shows the position of the cut-off at 300 nm.

Figure 4

Illustration of photoinduced hydrophilic behavior of two-sided ZrO₂ coatings on different substrate materials: (a) quartz support, (b) glass support. Yellow arrow means sunlight. Red arrow indicates the visible part of sunlight. Both sides, towards and away from the sun, of the ZrO₂ coating on quartz support are superhydrophilic. The back side of the ZrO₂ coating on glass support is not superhydrophilic in contrast to its front side.