Light-Controlled ZrO2 Surface Hydrophilicity

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.


Results
The initial state of zirconium dioxide polycrystalline films was characterized by a water contact angle of 12° ± 1° (Fig. 1a,b -image 1). After irradiation by UV-light with wavelengths shorter than 300 nm the surface became superhydrophilic characterized by a contact angle less than 5° ± 1° (Fig. 1a,b -image 2). At the same time, irradiation of the initial surface state by light with wavelengths longer than 300 nm results in a significant increase of the water droplet contact angle reaching the value of 24° ± 1° (Fig. 1a,b -image 3). These variations of the surface hydrophilicity correlate with alteration of the relative signals of O1s in XPS spectra (for details see "Supplementary Information"). In both cases the final surface states after irradiation are reproducible and preserve their hydrophilic state for several hours.
We found that the effect of spectral composition of actinic light on the surface hydrophilicity of zirconia can be reversibly switched from a highly hydrophilic state to a more hydrophobic state. Figure 2 demonstrates the repeatable change of the water contact angle on a ZrO 2 surface as a result of the alteration of the actinic light spectral composition. In Fig. 2, for simplicity, we present only six cycles of photoinduced switching of the surface hydrophilicity, each cycle consisted of the irradiation by light with λ < 300 nm for 15 minutes and the irradiation by light with λ > 300 nm for 15 minutes. In our experiments, the number of performed cycles reached up to 20 with good reproducibility. Thus, the irradiation in different spectral regions turned the surface of zirconium dioxide film to a superhydrophilic state under irradiation by light with λ < 300 nm, and to a more hydrophobic under irradiation by light with λ > 300 nm, and both states can be repeatedly switched from one to another by alteration of the spectral range of ZrO 2 photoexcitation.
It is important to note, that photoexcitation with wavelengths shorter than 300 nm corresponds mostly to the fundamental absorption of zirconia (E g ~ 5.0 eV, λ < 250 nm) while irradiation with light λ > 300 nm conforms Scientific RepoRts | 6:34285 | DOI: 10.1038/srep34285 to extrinsic absorption by defects such as Zr 3+ and F + -center electronic states in ZrO 2 . This assumption is confirmed by measurements of the photoluminescence excitation spectrum (λ em = 430 nm) presented in Fig. 3 and is in good accordance with earlier spectroscopic and luminescence results 11 .

Discussion
Luminescence is initiated by intrinsic excitation at energies above 5.0 eV, and by the extrinsic excitation complex band formed by several absorption single bands within the spectral region from ~5 to ~3 eV 11 . Luminescence centers are suggested to be oxygen anion vacancy related defects 11-13 , (V a ), which behave as deep electron traps:   Accordingly, the amphiphilic behavior of zirconia films under light of different spectral composition can be explained by the formation of different active surface states under photoexcitation in the intrinsic and extrinsic absorption regions. In the case of band-to-band excitation, illumination results in the superhydrophilic state. Irradiation in the extrinsic ZrO 2 region produces a more hydrophobic surface.
It is important to also note that for two-sided ZrO 2 coatings, the substrate material plays a significant role in the photoinduced alteration of the surface hydrophilicity for each side. For example, let us consider this effect for ZrO 2 nano-coatings on quartz and glass supports. The transmittance spectra of both these supports are presented in the "Supplementary Information". Both sides of two-sided ZrO 2 films on quartz supports demonstrate an identical photoinduced hydrophilic behavior (Fig. 4a) whatever the spectral composition of actinic light is. UV-light is transmitted through the fused quartz support without absorption, and the nano-coating on both sides becomes hydrophilic. At the same time, the glass support cuts light with λ < 300 nm, and therefore, only one side is irradiated in the intrinsic spectral region while the other side is excited in the extrinsic absorption spectral region only.  This results in different hydrophilic behavior of the front and back sides of two-sided ZrO 2 films on glass support (Fig. 4b): one side transforms to the superhydrophilic state, another to the more hydrophobic state.
The observed light-controllable reversible and reproducible switching of hydrophilicity opens new possible applications for the ZrO 2 based materials. One of them is the self-cleaning and antifogging effect under ultraviolet irradiation or the raindrop effect under visible light illumination. Another potential application can be oil-water separation. Irradiation by light with wavelengths shorter than 300 nm increases an oil-repellent property of zirconia or zirconia-containing coatings, membranes, and meshes, thus increasing the effectivity of separation or filtration processes of water from organics. There is a commercial ceramic membrane known as a Ceramesh (a composite of a metal mesh and a zirconia ceramic) that is already used for the separation of organics from raw water and water filtration 14 . Our findings could considerably improve the performance of such materials.
The thus synthesized ZrO 2 films were formed by nanoparticles with crystallite size of about 10-15 nm. The film thickness estimated by electronic microscopy was equal to 80-90 nm. Film surface smoothness was determined by AFM to be ± 2nm. Raman spectral analysis detected the presence of two ZrO 2 phases in the ZrO 2 coating -tetragonal and monoclinic. XRD analysis is in accordance with the Raman spectroscopic data. The contents of tetragonal and monoclinic phases are 72 ± 3 wt.% and 28 ± 3 wt.%, respectively. Detailed data on the ZrO 2 coating characterization are presented in the "Supplementary Information".
The water contact angle measurement and the study of photoinduced hydrophilic behavior of ZrO 2 surface. The water contact angle (Θ ) values were measured using a Theta Lite optical tensiometer (Biolin Scientific, Finland). Ultrapure water has a pH value of 5.5.
To obtain reproducible initial states of the ZrO 2 surfaces, we used a special procedure developed in our laboratory and described elsewhere [15][16][17] (see also "Supplementary Information") to obtain a hydrated surface which is characterized by a water contact angle value of 12.0° ± 1°. This state could stay unchanged during several hours at ambient conditions and was used as the initial state for the subsequent set of photo-experiments.
For irradiation in the intrinsic and extrinsic ZrO 2 regions, a 30-W deuterium lamp DDS-30 (LOMO) and a high-pressure 120-W mercury lamp (LOMO) were used, respectively. The cut-off filter at 300 nm (LOMO) was used to extract the extrinsic ZrO 2 region from the light of the Hg lamp.