Reactively-sputtered super-hydrophilic ultra-thin titania films deposited at 120 °C

We investigate super-hydrophilic TiO2 (titania) films for concentrated solar-thermal power applications. Reactive magnetron sputtering has been used to deposit 8 to 12 nm thick titania thin films onto borosilicate microscope glass slides, low-Fe extra-clear architectural glass, or Si(100) wafers with a 500 nm thick thermal SiO2 layer. The effects of deposition temperature and O2 fraction of the O2/Ar working gas were investigated. We demonstrate the importance of the O2 fraction for obtaining optically transparent, super-hydrophilic (contact angle below 1°) thin films. In particular, we show that as the O2 fraction increases, contact angle decreases, obtaining super-hydrophilic titania thin films at deposition temperatures as low as 120 °C. Our work enables to develop low thermal budget cost-efficient industrial synthesis processes, paving the way for commercial applications.

In this work, we focus on ultra-thin (8 to 12 nm thick), optically transparent titania films for CSP applications.Our main focus is to develop a thin film synthesis process compatible with industrial large-area and cost-efficient coating processes.Although there is a variety of titania thin films synthesis methods, including chemical vapor deposition and sol-gel synthesis, physical vapor deposition-particularly magnetron sputtering [12]-has shown advantages for self-cleaning glass applications and it is the synthesis method of choice for this study.Specifically, our main achievement was to develop superhydrophilic (close to zero contact-angle) titania thin films, at low deposition temperatures (120 °C).This paves the way for developing low thermal budget industrial synthesis, thus lowering the manufacturing cost and allowing for wide commercial application.It should be noted that although we used a lab-scale sputter deposition system (40 mm × 40 mm substrates), in our ongoing work the deposition recipes have been upscaled using an in-line sputter deposition system, which allows homogenous deposition on 300 mm × 300 mm substrates.Furthermore, on industrial scale, large area deposition of sizes up to 2 m × 3 m is very common for coating of architectural or display glass.

Experimental details
Thin films were deposited using an ultra-high vacuum (base pressure 1.5•10 −9 Torr) magnetron sputtering system in diode configuration (AJA International Inc. 2200 V).DC reactive magnetron sputtering was employed.A 99.99% pure metallic 2 inch diameter and 6 mm thickness Ti target from Testbourne B.V. was used as a source and the applied power density was 10 W cm −2 ; the power supply was run in power-controlled mode.The chosen power density provides a suitable film growth rate (see table 1) without thermally overloading the target.The distance between target and substrate was kept constant at 20 cm.The sputtering working gas was a O 2 /Ar mixture.Pure (5 N) Ar and O 2 were introduced into the sputtering chamber by two separate flow controllers.The Ar flow was varied between 80 and 100 sccm and the O 2 flow between 0 and 20 sccm.The reported O 2 fraction is defined as the ratio of O 2 flow to the total gas flow.Prior to film deposition, the Ti target was pre-sputtered in pure Ar for 30 min, in order to obtain a pure metallic surface.By monitoring target voltage during deposition, we determined that the process is repeatable when employing the same process parameters.The resulting growth rate as a function of O 2 percentage is shown in table 1.The growth rate has been determined using a Filmetrics Profilm3D optical profiler with white light interferometry.The substrates were three different types of commercially available SiO 2 glass: borosilicate microscope glass slides (1 mm × 25 mm × 25 mm from Velab), low-Fe extra-clear architectural glass (4 mm × 4 cm × 4 cm from Guardian Glass), or Si(100) wafers with a 500 nm thick thermal SiO 2 layer (0.5 mm × 10 mm × 10 mm from BT Electronics).The pressure of the working gas mixture was set to 7 mTorr, as it has been found that this value favors the growth of anatase films [13] (see Supplementary Information).During deposition the substrates were rotated at 80 revolutions per minute in order to ensure homogeneous film deposition.Films were deposited at substrate temperatures ranging from 120 °C to 500 °C.
X-ray diffraction (XRD) structural analysis was performed using a Siemens D500 diffractometer in Bragg-Brentano geometry with Cu-K a radiation, in steps of 0.03°and counting time 12 s step −1 .θ-2θ scans were obtained in the 20°to 32°range.
Atomic force microscopy (AFM) was employed to investigate the film surface morphology, using a NT-MDT scanning probe microscope in non-contact mode and commercial SPM probes.Raman measurements were performed with a Renishaw In-Via Reflex dispersive micro-Raman spectrometer with less than 2 cm −1 resolution and a diode laser, emitting at wavelength λ = 514.4nm, as the excitation source.A Leica DMLM microscope with a ×100 objective lens was used to focus the laser beam to 1 μm diameter at the samples' surface.
The optical properties of the samples were examined using a Hitachi 3010 UV vis −1 spectrophotometer, equipped with a 60 mm diameter integrating sphere and BaSO 4 as a reference.Spectroscopic ellipsometry measurements were performed within the 350-1000 nm range using a J A Woollam Inc. M2000F rotating compensator ellipsometer (RCE™) running the WVASE32 software at an angle of incidence of 75.14°.
The hydrophilicity of the films was evaluated with a CAM 100 Contact Angle Meter (KSV Instruments Ltd.), equipped with appropriate software.Two different series of experiments were performed for the contact angle (CA) determination: in the first series, the contact angle was measured immediately after film deposition.In the second series, a 5 min UV-A light (intensity ∼2.6 mW cm −2 ) treatment was performed before the CA measurements.In most cases the measurements were repeated three times (due to restrictions arising from the samples' available surface) and the mean values are presented.

Results and discussion
3.1.Variation of substrate temperature All the studied films are optically transparent in the visible and near-IR spectra (see figure 2 of supplementary information).First, the effect of deposition temperature on the crystal structure of the films has been studied.Figure 1 presents XRD measurements performed on 10 nm thick films deposited onto microscope glass slides.We varied the deposition temperature from 120 °C to 500 °C, while maintaining a constant O 2 fraction of 10%.No distinct Bragg peaks are detected for deposition temperatures between 120 °C and 350 °C suggesting that the films are primarily amorphous.Also, increasing the O 2 fraction to 20%, results in the formation of an    corresponding average grain size increases from 70 nm to 90 nm, as the deposition temperature rises from 120 °C to 500 °C.The above trends are in accordance with previous reports [15], where a significant increase in roughness and grain size is observed only at temperatures above 600 °C.It should be noted that a bare microscope glass slide has an RMS roughness equal to 0.5 nm (see supplementary information).
Figure 3 summarizes the findings obtained by contact angle measurement for various samples; selected pictures can be found in the supplementary information.Contact angle measurements have been performed on 10 nm thick films deposited onto low-Fe extra-clear glass which is usually employed for CSP applications.For a constant O 2 fraction of 10%, the contact angle decreases from 64°to 30°as the deposition temperature rises from 120 °C to 400 °C.Recent molecular-dynamics simulations [16] have shown that among the three possible TiO 2 crystal structures-anatase, brookite, and rutile-anatase has the best hydrophilicity, exhibiting the lowest contact angle; in addition, it has been reported that surface morphology also affects the surface wetting of anatase, where the contact angle rises with increasing surface roughness.Taking into account our abovereported XRD and AFM results, we conclude that the observed contact angle decrease is accounted for by the fact that the anatase crystal structures appears at 400 °C.The effect of the slightly increased surface morphology may also contribute to the observed hydrophilicity enhancement, although to a lesser degree.Furthermore, in accordance with previous reports [17,18], we also observe photo-induced hydrophilicity.In particular, figure 3 indicates that UV irradiation decreases the contact angle by values between 18°and 29°, reaching a maximum decrease for the film deposited at 400 °C, which has the anatase crystal structure [19] (see figure 1).

Variation of oxygen fraction
In our quest to minimize the manufacturing energy cost for the deposition of titania thin films, we investigated the effect of the O 2 fraction in the O 2 /Ar working gas on film microstructure and properties.It should be noted that although the films are expected to be sub-stoichiometric for 10% O 2 , a stoichiometric TiO 2 composition is expected for 20% O 2 content, a finding which is corroborated by spectroscopy ellipsometry characterization of our samples [20] (presented below).
Apparently, upon increasing the O 2 percentage to 20% -while maintaining the deposition temperature at 120 °C -the film remains amorphous (see figure 1).However, the O 2 fraction increase has a significant effect on the surface morphology of the films, in accordance with previous reports on the significance of deposition parameters on the titania films' microstructure [21].In figure 2, the right image shows the surface morphology of a film deposited at 120 °C at a 20% O 2 content.Surface morphology appears to be smoother compared to 10% O 2 , with an RMS roughness of 0.8 nm and an average grain size of 35 nm.It is important to emphasize that the observed decrease in surface roughness with increasing O 2 fraction is in contrast to previous reports [22]; however, in [22] the effect of very low O 2 fractions, ranging from 0% to 4%, has been investigated.
The most striking result of increasing the O 2 fraction is the drastic reduction of the contact angle.First, in figure 3 we observe that for a constant deposition temperature of 400 °C, the contact angle slightly decreases from 37°to 30°, when the O 2 fraction increased from 5% to 10%.However, a dramatic decrease to a contact angle below 1°is observed when the O 2 fraction increases to 15%.Even more interestingly, this extremely low contact angle is observed for films deposited at 120 °C, if the O 2 content is 15%, or higher.
Taking into account the above findings, we identify the 120 °C deposition temperature and a 20% O 2 fraction as the ideal thin film synthesis parameters.Thus, we synthesized a 8 nm thick TiO 2 film on SiO 2 (500 nm)/Si(100) and characterized it using spectroscopic ellipsometry (SE).We employed this particular substrate due to its very low surface roughness (see supplementary information) to facilitate SE measurements.Typical fitting results of experimental and calculated psi and delta are seen in figure 4. Experimentally recorded psi and delta spectra were initially analysed using a Cauchy model.An excellent agreement between experimental and theoretical results is observed.
In figure 5, wavelength variations of the real (left) and imaginary (right) part of the refractive index of the 8 nm thick titania film is shown.We determine the refractive index to be 2.47 at 550 nm, higher than those prepared by other techniques [23].Usually, a higher refractive index implies a higher packing density [24], or a metal-rich film.Here, we infer that the film is not metal-rich, because of the low extinction coefficient, which indicates an ideal stoichiometry of the oxide film [20].
Finally, the optical band gap of the TiO 2 thin films can be determined from Tauc plots [25].The spectral behaviour of the absorption coefficient (α) was calculated from the optical measurement and afterwards, the relationship between (α * E) 1/2 and the photon energy was plotted (figure 6).The TiO 2 thin film band gap can be calculated by extrapolating the straight line of the relationship between (α * E) 1/2 and the photon energy.The  value of E g for the 8 nm TiO 2 thin film was found to be 4.1 eV.This value is slightly higher than previous results obtained for TiO 2 thin films prepared by the sol gel method [26] and by sputtering [27], due to the small thickness of the film [26].
The above results emphasize the importance of the O 2 fraction in the O 2 /Ar working gas during reactive sputter deposition of titania for obtaining super-hydrophilic surfaces.In the past [14][15][16][17][18][19], the material crystal structure has been shown to be a critical parameter affecting surface contact angles; in particular, anatase titania films were required for very low contact angle surfaces.However, it should be made clear that previous studies reporting on super-hydrophilic titania thin films concern RF sputter deposition using ceramic TiO 2 targets [13, 28] and a pure Ar working gas.This approach results in sub-stoichiometric titania films, where the reported subsequent annealing in air, at temperatures higher than 400 °C, is needed to achieve hydrophilic or super-hydrophilic surfaces, probably due to oxygen incorporation into the film from the ambient air and the alteration of the titania stoichiometry.Furthermore, previous reports show that by treating sol-gel deposited thin titania films by DC-glow discharge air plasma, super-hydrophilicity can be obtained [29].Although, the authors attribute this to the increase of the surface roughness-a result that contradicts our findings-the effect of the O 2 plasma on the stoichiometry of the surface has also been reported to contribute to the large decrease of the wetting contact angle.Taking the above into account, we believe that surface chemistry is of major importance for obtaining super-hydrophilic titania surfaces.Increasing the oxygen fraction might not only affect the physical structure of the film but also its chemical nature.A higher oxygen fraction may lead to an increase in the number of hydroxyl groups on the surface, thus increasing the surface coverage by chemisorbed water molecules to create an extremely thin water overlayer, ultimately making the surface super-hydrophilic [30].

Conclusions
Previous studies on sputtered titania films report that superhydrophilic properties may be obtained only after deposition or post-deposition annealing at a temperature of 400 °C, or higher, or after UV irradiation.In our work, in contrast to previous reports, superhydrophilic (contact angle below 1°) titania films have been obtained at the low deposition temperature (120 °C), without the need for UV irradiation.Our work shows that besides achieving the anatase crystal structure, another critical parameter is the working gas O 2 fraction.Reactive sputter deposition in a working gas with high O 2 fraction (15%, or higher), allows obtaining super-hydrophilic titania films, even though they have an amorphous crystal structure.We determine that the optimum conditions for reactively sputtered TiO 2 films is an oxygen fraction of 20% and a deposition temperature of 120 °C; furthermore, the optimum working gas pressure has been determined to be 7 mTorr.Finally, the optimum thickness is determined to be 10 nm.We believe that our findings greatly enhance the potential of titania films to be implemented in commercial applications.
amorphous film.At 400 °C, the characteristic Bragg peak of the (101) crystallographic planes of the anatase crystal structure appears at 25.30°, indicating at least a partial crystallization[14].Increasing the deposition temperature to 500 °C results in the appearance of the rutile crystal structure, as revealed by the characteristic Bragg peak at 27.5°, while the anatase phase disappears.Apparently, for 10% O 2 fraction, there is a narrow temperature range 'window' where the desired anatase phase may be obtained.It should be noted that for XRD measurements we employed the Jade database and JPDS Cards No.21-1272 for anatase and No.21-1276 for rutile.In addition, we have chosen the 20°to 32°angle range as the most intense anatase and rutile peaks appear at approximately 25.4°and 27.5°; furthermore, the brookite phase-if present-would have appeared at approximately 31.0°.The surface morphology of the films has been characterized using AFM.Measurements were performed on 10 nm thick films deposited onto microscope glass slides, at temperatures ranging from 120 °C to 500 °C, and O 2 fractions of 10% or 20%. Figure 2 shows three indicative AFM images.It appears that for an O 2 content of 10%, minor changes on surface morphology occur at the studied deposition temperatures.The root meansquare (RMS) roughness slightly increases from 1.2 nm (left image) to 1.3 nm (center image) and the

Figure 1 .
Figure 1.X-ray diffractograms of 10 nm thick titania films deposited at various temperatures onto microscope glass slides.The O 2 content was 10% or 20%.Diagrams are shifted in the vertical axis for clarity.

Figure 3 .
Figure 3. Contact angle determined for various 10 nm thick TiO 2 films deposited onto low-Fe extra-clear glass.The numbers on the horizontal axis denote deposition temperature-O 2 fraction.Contact angles have been measured in the as-deposited state (without UV) and after UV treatment.

Figure 4 .
Figure 4. Experimentally determined and theoretically calculated wavelength variations of psi and delta of an 8 nm thick titania film deposited at 120 °C and 20% O 2 onto SiO 2 (500 nm)/Si(100) substrate.

Figure 5 .
Figure 5. Wavelength variations of the real (left) and imaginary (right) part of refractive index of the sametitania film as in figure 4.

Figure 6 .
Figure 6.Tauc plot of the same titania film as in figure 4 to determine its E g value.

Table 1 .
Titania growth rate as a function of O 2 fraction.The power density and working gas pressure were kept constant at 10 W cm −2 and 7 mTorr, respectively.