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Integrated Nano- and Macroscale Investigation of Photoinduced Hydrophilicity in TiO2 Thin Films Corrado Garlisi, Gabriele Scandura, Giovanni Palmisano, Matteo Chiesa, and Chia-Yun Lai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03756 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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Integrated Nano- and Macroscale Investigation of Photoinduced Hydrophilicity in TiO2 Thin Films Corrado Garlisi,1§ Gabriele Scandura,1§ Giovanni Palmisano,1 Matteo Chiesa,*2 Chia-Yun Lai*2 1

Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology, PO BOX 54224, Abu Dhabi, UAE. 2 Laboratory for Energy and NanoScience (LENS), Masdar Institute of Science and Technology, PO BOX 54224, Abu Dhabi, UAE. Email: [email protected]; [email protected] § These authors contributed equally.

ABSTRACT The hydrophilicity of Titanium dioxide has been investigated for films, deposited on glass by e-beam evaporation, being exposed to UV radiation and subjected to a thermal annealing. Wettability alteration has been showed to depend upon both treatments, and insights on how to introduce a more stable hydrophilicity on these films have been presented for the sake of boosting their commercial value. Observations from multiple length scales to assess the wetting behavior of as-deposited and high-temperature annealed samples were assessed through macroscopic measurements, i.e. water contact angle measurements, showing that the annealed crystalline samples, treated at 500°C, are much more hydrophilic (SCA ≈ 20°) than as-deposited TiO2 films (SCA ≈ 90°), and the nanoscopic experiments performed by amplitude modulation (AM) atomic force microscope (AFM) indicated that this increased hydrophilicity is related to an enhanced adhesion force and surface energy, resulting upon partial crystallization of TiO2 and the consequent formation of crystals on its surface, rather than being related to morphologic differences. XRD and Raman measurements have highlighted, on one other hand, that the crystallinity of TiO2 film is crucial in determining its hydrophilicity, in good agreement with the AFM study. The results indicated also that, after irradiation, the samples treated at 500°C preserve their hydrophilicity for a significant time compared to previous studies.

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INTRODUCTION Titanium dioxide (TiO2) has attracted substantial research interest due to its unique properties which make it suitable for a plethora of applications in the fields of photocatalysis,1-4 dye-sensitized solar cells,5 gas sensing,6-7 etc. It has been commercially exploited in self-cleaning8-10 and antifogging coatings,11 a function enabled by the combined effect of photocatalysis and photo-induced, specifically UV-induced, hydrophilicity. The UVinduced hydrophilicity is believed to result from the increase in the amount of hydroxyl groups on the TiO2 surface, which results in a change of the interfacial energy between the solid surface and the liquid.12 In addition, UV light induces the generation of a large number of surface oxygen vacancies and dangling bonds which facilitate the dissociative adsorption of molecular water, thus increasing the surface extent of hydrophilic regions.13,14 Water drop contact angle measurements have been widely used to investigate modifications on the macroscopic wettability of TiO2 during and after UV exposure; nevertheless, some crucial aspects regarding the nanoscale mechanisms of the photoinduced wettability and the related changes in surface chemistry are difficult to assess univocally by means of macroscopic investigations. In particular, previous studies of TiO2 wettability15-16 have struggled to decouple the effects of surface chemistry (mainly crystals arrangement and hydroxylation) from those of morphology. Atomic Force Microscopy (AFM) provides a versatile and convenient tool to gain a deeper physical insight into the nanoscale processes taking place at TiO2 surface, allowing direct investigation of surface chemistry modification independently of morphological effects. In previous AFM study of TiO2 films,17 artifacts caused by “jump-to-contact” behavior18 have complicated the interpretation of the results. This challenge may be avoided by the recently developed


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dynamic AFM,19 or amplitude modulation (AM) AFM, force spectroscopy technique that avoids the “jump-to-contact” phenomenon due to the energy stored in the cantilever competing with tip-sample interactions. A superior understanding of the wettability properties of TiO2 films could contribute to improving TiO2-based self-cleaning coatings such as those already offered by such companies as Pilkington ActivTM, SSG BiocleanTM, and SuncleanTM. These coatings, when exposed to UV radiation, break down organic contaminants via photocatalytic action and additionally, due to their UV-induced superhydrophilicity, encourage water to spread evenly over the surface and wash away the loosened dirt.20 Such coatings, however, rapidly lose their superhydrophilic properties when UV radiation is not present, constituting a substantial hindrance for indoor applications where only visible light is present. Therefore, it would be highly desirable, from an application perspective, to develop means of inducing permanent hydrophilicity in the TiO2 film, which is not dependent on UV illumination. One possibility for improving the wettability of the TiO2-based coatings is high-temperature annealing treatment, which is widely used in thin film fabrication to reduce strain generated during the fabrication process and to modify film crystallinity, morphology and adhesion to the substrate. As a result of these modifications, annealing may induce desirable changes in the macro- and nanoscale wetting properties of the TiO2 surface. In this study, both nanoscopic measurements, i.e. AM AFM force spectroscopy, and macroscopic measurements, i.e. water static contact angle (SCA), have been employed to develop a more comprehensive understanding of TiO2 thin film surface modifications with an eye towards improving their application potential for self-cleaning surfaces. Macroscopic measurements have been carried out by previous studies, led from other research groups.21,


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22

However, they focused on doped TiO2 thin film deposited by RF magnetron sputtering. In

the present work, a comprehensive multi-length scale investigation has been applied on non-doped TiO2 thin films casted by e-beam evaporation.

EXPERIMENTAL Sample preparation TiO2 films were deposited on soda-lime glass substrates 25x75 mm (Sigma-Aldrich) by ebeam evaporation. Glass substrates were cleaned in an ultrasonic bath with acetone and isopropanol in two successive 10-minute steps. 1-3mm pellets made of 99.9% pure TiO2, provided by Plasmaterials, were used as source materials in the Temescal BJD-2000 e-beam evaporation system. The deposition chamber was evacuated to a base pressure of 3.0 x 10−6 Torr and the substrates were rotated at 40 rpm during the deposition. The electron gun voltage and the deposition rate were 10kV and 1 Å s−1 respectively in order to obtain a final thicknesses of 250 nm. After deposition, films were annealed in air using the following procedure: heating up to 475°C (ramp rate of 10°C/min), 5 min at 475°C, heating up to 500°C (ramp rate of 2.5°C/min), 4 hours at 500°C. Another batch of films were annealed at 350°C with a similar temperature program. For the sake of clarity, we labeled ‘as-deposited TiO2’ the samples not undergoing a thermal treatment, whereas ‘350-TiO2’ and ‘500-TiO2’ refer to the samples that were annealed at 350°C and 500°C, respectively, after e-beam evaporation on the glass slides.


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Contact angle measurement The contact angle measurements were performed by a Kruss Easy Drop Contact Angle analysis machine and the standard software supplied by Krüss was used for the evaluation of the data. All the samples were cleaned in acetone and then in ethanol in two successive 5-minute steps and finally dried under Argon flow. Samples were then heated at 50°C for 30 min in order to remove volatile organic residues. Before measuring, thin films were kept in the dark for 20 days. SCA was measured using 5 μL droplets and five consecutive measurements were performed on each sample to report a reliable average value. In order to study the evolution of the SCA under UV light, samples were irradiated by a 250W Mercury UV-lamp connected to an optical fiber for 150 min. The average values of radiation intensity reaching the film surface, measured with a Delta Ohm 9721 radiometer and the matching probes, were 4.9 W m−2 in the range 200-280 nm, 21 W m−2 in the range 280-315 nm, 6.2 W m−2 in the range 315-400 nm and 1.9 W m−2 in the range 450-950 nm. Control experiments on bare glass have been performed, showing no detectable change caused by irradiation, as expected.

X-ray diffraction The crystalline structure of the films was characterized by X-Ray diffraction (XRD) measurements, with CuKα radiation (1.5418 Å) at power settings of 45 kV and 40 mA. The analysis was performed by a Panalytical Empyrean system. Diffraction patterns were recorded in the range of diffraction angles 2θ from 20° to 60°, with a grazing angle of 3° and a scan rate of 0.075°/min.


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Raman spectroscopy Raman spectroscopy was carried out by Witec Alpha 300R equipment, with an excitation wavelength of 532 nm and a laser power of ca. 75 mW. Scans were taken over an extended range (100-800 cm-1) with 3s integration time and 200 accumulations.

Atomic force microscopy A Cypher AFM from Asylum Research was operated in amplitude modulation (AM) mode and standard AC240TS cantilevers (k ≈ 2N/m, Q ≈ 100, and f0 ≈ 70 kHz) were used for all the AFM experiments. For sample surface scanning, scanning rate of 0.8 Hz and oscillation amplitude of ∼ 5 nm were used. For the force spectroscopy technique, amplitude A and phase Φ versus tip-sample separation distance d, i.e. APD, curves were recorded. To reconstruct the tip-sample interaction force, the Sader-Jarvis-Katan formalism23 was employed (details presented elsewhere24). Since it is well- known that the tip radius R significantly affects the tip-sample interaction force, R was monitored in all experiments with critical amplitude (Ac) method25 to make sure that R remains constant throughout the experiment. The samples were analyzed before and after 150 min of exposure to same UVlight used during the contact angle experiment. The second mode parameters for AC240TS cantilevers are k ≈ 80N/m, Q ≈ 400, and f0 ≈ 420 kHz. For operating in the attractive regime to obtain Hamaker coefficients, we set the first mode free amplitude to ~0.5Ac and set-point to ~70% of first mode free amplitude. As for operating in the SASS regime, we set the first


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mode free amplitude according to the amplitude-distance curves where regions of negative slope were observed.

RESULTS AND DISCUSSION TiO2 surfaces are first examined with AFM imaging. Figure 1 shows the AFM topography and phase images for as-deposited (Fig. 1a,d), 350-TiO2 (Fig. 1b,e) and 500-TiO2 (Fig. 1c,f) samples. As-deposited TiO2 and 350-TiO2 present similar morphology, while 500-TiO2 surface displayed some structures that were further analyzed with XRD measurements. Figure 2a compares the XRD patterns of the TiO2 films before and after annealing at 350 and 500°C. The film becomes crystalline after annealing at 500°C, showing the peaks characteristic of anatase phase26 located at 2θ = 25.33°, 37.82°, 48.08°, 55.12°, which can be indexed as the (101), (004), (200) and (211) planes, while the as-deposited and 350-TiO2 thin film present no such peaks, pointing to a quite amorphous structure. Similar observations can be deduced by the AFM phase channel (Fig. 1d-f).


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Figure 1. AFM topography image of as-deposited-TiO2 (a), 350-TiO2 (b), 500-TiO2 (c) and AFM phase image of as-deposited-TiO2 (d), 350-TiO2 (e), 500-TiO2 (f).

The Raman spectroscopy investigation (Fig. 2b) corroborates that anatase is the only polymorph of TiO2 in the samples after annealing at 500 °C.27 Nevertheless, the film annealed at 350 °C owns a fair hydrophilic character and this fact is proved by the contact angle measurements after UV light exposure (Fig. 3). This improved performance of 350TiO2, compared to the as-deposited sample, is a clue of the nucleation of the first anatase nanocrystals at 350 °C, which are however below the detection limits of XRD and Raman.


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Figure 2. (a) XRD diffractogram of as-deposited TiO2, 350-TiO2 and 500-TiO2 (a), the dashed black diffractogram is a reference XRD of pure anatase. (b) Raman spectra of as-deposited TiO2, 350-TiO2 and 500-TiO2.

Figure 3 shows that the SCA of the as-deposited films before UV-irradiation is 96±10°, revealing the hydrophobic character of the sample. In contrast, the 500-TiO2 is moderately hydrophilic with SCA = 20±7°. The 350-TiO2 had an intermediate behavior (SCA = 45±8°). The time-evolution of the contact angle under UV irradiation is shown in Fig. 3a. After a 150 min exposure period to UV-light, the water droplet spreads out on the film surface almost completely, as expected, yielding small contact angles of 9±2°, 7±2° and 6±2° for the asdeposited TiO2, 350-TiO2 and 500-TiO2, respectively. Similar trends were observed with TiO2 films prepared by different deposition techniques such as MOCVD (metal organic chemical vapor deposition),28 sol-gel process,29 radio frequency magnetron sputtering.30 However, the temporary range needed for those samples to switch from hydrophilic back to hydrophobic after discontinuing UV irradiation, ranged from few hours to a maximum of 3 days. Conversely, the samples presented in this study have much more stable hydrophilicity characteristics after being exposed for 150 minutes under UV irradiation. This is confirmed by the SCA measurements that equal 18±5°, 35±5°and 60±6° for 500-TiO2, 350-TiO2 and as-


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deposited TiO2, respectively, after 3 months of storage in dark conditions. Thus, the result obtained with 500-TiO2 is improved in terms of long lasting hydrophilicity.

Figure 3. (a) Time-evolution under UV irradiation of the SCA for as-deposited TiO2, 350-TiO2 and 500TiO2, along with SCA after 3 months of storage in the dark; (b) SCA images of the three samples at t=0 min and after 150 minutes.

It is well known that material wettability is affected by morphology and surface chemistry3134

. Therefore, surface root-mean-square (RMS) roughness of TiO2 thin film samples has been

examined, and the figures found before and after UV irradiation are 3.7±1.1 nm and 4.1 ±


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0.6 nm for as-deposited TiO2, 4.8±3.8 nm and 4.1±3.2 nm for 350-TiO2, 13.6±1.5 nm and 11.3 ± 1.8 nm for 500-TiO2. The small difference in the RMS roughness value before and after UV treatment for these samples implies that the wettability alteration of TiO2 thin film is not caused by morphological changes. However, since 500-TiO2 presents a much higher roughness than as-deposited and 350-TiO2, to fully assess the effect of roughness on wettability, the following Wenzel’s equation has been employed to relate the smooth surface contact angle to the experimental one:35-36

= where θm is the experimentally measured contact angle, r is the roughness factor, and θY is the calculated contact angle for a perfectly smooth surface. We obtained a calculated θY of 96.3°, 45.1° and 26.5° for as-deposited, 350-TiO2 and 500-TiO2 samples before UV treatment and 11.3°, 9.3° and 18.5° after UV treatment, respectively. The calculated contact angle θY for as-deposited TiO2 and 350-TiO2 before and after UV treatment are within the SCA experimental measurements error, i.e. θY ∈ [θm–error, θm+error]. While for 500-TiO2, we noticed that before UV treatment, the difference between θY and θm is higher. This indicates that the higher roughness of 500-TiO2 film play a role in enhancing the hydrophilic properties of the surface. In other words, the more hydrophilic character of 500-TiO2 may due to the increased roughness of the surface, yet the wettability variation resulting from UV exposure is dependent on a significant change in surface chemistry causing TiO2 surface to become hydrophilic. Additionally, as mentioned before, the hydrophilic properties of TiO2 thin films are preserved for longer time in the absence of UV when the film has been treated at 500 °C.


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The variation in surface chemistry was validated by comparing the Hamaker coefficient values for all three samples before and after UV exposure. We excited the AFM probe with 2 frequencies so that we could obtain extra experimental observables to compute effective Hamaker coefficients37. The effective Hamaker coefficients are dependent on AFM tip radius, oscillation free amplitudes, set-points as well as the experiment environment. In order to avoid complication of this work, we normalized the change of Hamaker coefficients with the value before UV exposure. As presented in Fig. 4, we could see that after UV exposure, the Hamaker coefficient values all increased, with 94.9%, 155.6% and 52.7% for as-deposited TiO2, 350-TiO2, and 500-TiO2. To further investigate the reasons behind how annealed TiO2 could possess such properties, the amplitude versus distance curves recorded via AM AFM have been analyzed as shown in Fig. 5. The six cantilever oscillation amplitude (A1) versus distance (Zc) curves represent asdeposited (Fig. 5a,d) and 350-TiO2 (Fig. 5b,e) and 500-TiO2 (Fig. 5c,f). In Fig. 5d,e,f, it can be seen that the curves exhibit a negative slope at small Zc after UV exposure; it has previously been established that this feature indicates the presence of a nanoscale water layer on the sample surface,38-39 and thereby demonstrates the nanoscale hydrophilicity of the UVtreated surfaces.


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Figure 4. Hamaker mapping of as-deposited-TiO2 (a), 350-TiO2 (b), 500-TiO2 (c) before UV irradiation, and of as-deposited-TiO2 (d), 350-TiO2 (e), 500-TiO2 (f) after UV irradiation.

Figure 5. The AFM probe oscillation amplitude versus tip-sample distance curves for as-depositedTiO2 (a), 350-TiO2 (b), 500-TiO2 (c) before UV irradiation, and for as-deposited-TiO2 (d), 350-TiO2 (e), 500-TiO2 (f) after UV irradiation.


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Figure 6 displays reconstructed tip-sample interaction force versus distance profiles for the as-deposited and annealed samples prior to UV exposure. The 500-TiO2 sample presents a larger plateau than the as-deposited TiO2 and 300-TiO2 in the attractive region of the force profile, demonstrating the presence of a thicker nanoscale water layer on the annealed sample surface.19 In addition, the minimum force (i.e. adhesion force, F) reads 0.76 ± 0.25 nN for as-deposited TiO2, 0.62 ± 0.28 nN for 350-TiO2 and 2.82 ± 1.64 nN for 500-TiO2. The relationship between the adhesion force and the surface energy using the sphere-flat plane model is depicted as follow:40-41 = 4 where R is the AFM tip radius and γ is the surface energy. Since the AFM tip radius has been carefully kept unchanged throughout the data collection, a larger F yields a larger surface energy, and hence a smaller contact angle is expected. The results of 500-TiO2 samples having higher F are in line with the macroscopic contact angle measurements. The detailed adhesion force for all the samples before and after UV treatment is presented in Table I.


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Figure 6. Tip-sample interaction force profiles for as-deposited TiO2, 350-TiO2 and 500-TiO2 before UV irradiation.

Table I. Adhesion force for as-deposited TiO2, 350-TiO2 and 500-TiO2 before and after UV treatment.

As-deposited TiO2 350-TiO2 500-TiO2

Before Adhesion force (nN) 0.76 0.62 2.82

Standard error 0.25 0.28 1.64

After Adhesion force (nN) 0.84 1.71 6.84

Standard error 0.34 3.21 2.54

The force reconstruction method provided information on a single point of the studied sample. To map the force of adhesion for a larger region, the recently developed small amplitude small setpoint (SASS) method to acquire data for calculating the tip-sample interaction force while scanning has been exploited.39 This technique utilizes the AFM probe oscillating under the nanoscale water layer on the sample surfaces to obtain average adhesion force while scanning the sample. In other words, this mode of operation was carried out when the AFM probe oscillated in the negative slope region highlighted in Figure 5 with green circles and it could not be performed with absence of UV irradiation. Figure 7 shows average force maps for all the samples after the UV irradiation. As it can be seen in the figure, it is clear that all 3 samples exhibited heterogeneity at the nanometer scale. In addition, Fig. 7c shows that the larger TiO2 crystals, identified in the sample annealed at 500°C and highlighted by red circles, are characterized by a higher adhesion force due to their crystallinity and larger surface: both factors are thought to be responsible for the lower contact angles and the higher adhesion forces found anytime throughout the experiments for the 500-TiO2 samples.


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Figure 7. Average force maps for as-deposited-TiO2 (a), 350-TiO2 (b) and 500-TiO2 (c) after UV irradiation.

CONCLUSIONS An integrated nano- and macroscale inspection of wettability alteration in e-beam evaporated TiO2 thin films has been conducted via annealing at 2 different temperatures (350 and 500°C) and UV exposure. Via X-ray diffraction, Raman spectroscopy and AFM topography analysis, it has been shown that the TiO2 films undergo crystallization in the anatase polymorph after annealing at 500°C. Macroscopic wettability measurements highlight that 500-TiO2 (contact angle ∼ 20°) is substantially more hydrophilic than 350-TiO2 (contact angle ∼ 45°) and as-deposited TiO2 (contact angle ∼ 96°) without UV irradiation. This difference in wettability can be attributed to the difference in both the RMS roughness of the samples and adhesion force F, which is smaller for as-deposited and 350-TiO2 samples than 500-TiO2. The higher adhesion force of 500-TiO2, in turn, indicates that the surface energy is higher than the other 2 samples, leading to enhanced wetting of the surface. Nanoscopic observation using AFM provided indisputable evidence of an adsorbed water layer, indicating enhanced nanoscale wettability, on the 500-TiO2 sample before UV treatment as well as on UV-exposed samples, which were all shown to be highly hydrophilic. Recently-developed force mapping techniques determined that all samples were not


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homogeneous in terms of force of adhesion at the nanoscale, and the crystalline part of the 500-TiO2 sample exhibited an increased force of adhesion. Based on this, it was proposed that the hydrophilicity of the 500-TiO2 film is primarily a result of this partial crystallization and the consequent modification of the surface chemistry, rather than being a mainly morphology-dependent effect. Furthermore, annealing process as well as the annealing temperature prolong TiO2 film hydrophilic properties as contact angle measurements of 500-TiO2, 350-TiO2 and as-deposited TiO2 showed ∼18°, ∼35° and ∼60° respectively, after 3 months aging in the dark condition. With these results, the nanoscale mechanisms that determine the wetting properties of TiO2 films has been validated in terms of the improved hydrophilicity performance. The demonstration of the effectiveness of annealing above 350°C to induce UV-independent hydrophilicity, in particular, may suggest an alternative pathway to designing multipurpose indoor/outdoor coatings.

ACKNOWLEDGEMENTS Masdar Institute of Science and Technology is gratefully acknowledged for financial support (FA2014-000010). The Core Technology Platform at New York University Abu Dhabi is acknowledged for the XRD diffractions performed by James Weston.

REFERENCES

1. Nakata, K.; Fujishima, A. TiO2 photocatalysis: Design and applications. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2012, 13 (3), 169-189.


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