Photocatalytic TiO2 thin films synthesized by the post-discharge of an RF atmospheric plasma torch

Surface & Coatings Technology 289 (2016) 172–178

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Photocatalytic TiO2 thin films synthesized by the post-discharge of an RF atmospheric plasma torch Stéphanie Collette a,⁎, Julie Hubert a, Abdelkrim Batan a, Kitty Baert b, Marc Raes b, Isabelle Vandendael b, Alain Daniel c, Catherine Archambeau c, Herman Terryn b, François Reniers a a b c

Service de Chimie Analytique et Chimie des Interfaces (CHANI), Université Libre de Bruxelles (ULB), Boulevard du Triomphe 2, 1050 Brussel, Belgium Department of Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussel, Belgium CRM Group — Advanced Coatings & Construction Solutions (AC&CS), Allée de l'Innovation, 1, B57, Quartier Polytech 3, B-4000 Liège, Belgium

a r t i c l e

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Article history: Received 17 September 2015 Revised 29 December 2015 Accepted in revised form 25 January 2016 Available online 27 January 2016 Keywords: Titanium dioxide Plasma deposition Atmospheric post-discharge Photocatalysis

a b s t r a c t Thin films of titanium oxide (TiO2) are synthesized at room temperature by the post-discharge of an RF atmospheric plasma torch supplied with argon and oxygen. Vapours of titanium tetraisopropoxide (TTIP) precursor are injected in the post-discharge by an argon flow rate bubbling in the liquid precursor. Without any external substrate heating, the coatings are amorphous and characterized by a thin film upon which agglomerates can be observed. The annealing of the coatings at 450 °C is efficient to (partially) crystallize the TiO2 since bands characteristic of the TiO2 anatase structure are observed in Raman spectra. The films are super-hydrophilic and present excellent photocatalytic activity; two properties of particular interest for self-cleaning applications. Nevertheless, annealed coating presents a higher photocatalytic activity. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Numerous studies have focused on titanium dioxide films (TiO2) for its outstanding properties including biocompatibility [1–4], semiconductivity [5–6], anticorrosive [7], antibacterial [8–9], gas sensing [10] and photocatalytic activity [11–14]. The latter property is of great interest for industrial applications where “self-cleaning” surfaces are required [15–16]. Additionally to its ability to photodegrade organic pollutants, UV irradiation of titanium dioxide induces superhydrophilicity which facilitates the removal of any residual material [15,17–18]. Wang et al. [19] analysed the photo-induced superhydrophilicity of TiO2 which was explained by the unique character of TiO2 surface directly related to its microstructures. This mechanism has been investigated and found his origin in the desorption of H2O due to the heating effect under the UV irradiation and to the elimination of hydrocarbons by the photocatalytic oxidation [20]. Conventional techniques leading to the deposition of titanium dioxide films are sol–gel method [21], chemical vapour deposition (CVD) [22], magnetron sputtering [23] or plasma spraying [24]. Studies focusing on the deposition by plasma-enhanced chemical vapour deposition (PECVD) have been developed for few years [25–26]. However, most works have been done at low pressure and/or using TiCl4 as a precursor. ⁎ Corresponding author. E-mail address: [email protected] (S. Collette).

http://dx.doi.org/10.1016/j.surfcoat.2016.01.049 0257-8972/© 2016 Elsevier B.V. All rights reserved.

The use of titanium tetraisopropoxide (TTIP) as a precursor allows avoiding toxic by-products and chlorine contamination observed with TiCl4. Other alkoxides such as Ti(OEt)4 have also been used for the deposition of TiO2 films with low carbon content [27]. However, Ti(OEt)4 based compounds appear to have a lower photoactivity towards the degradation of pollutants such as the 4-chlorophenol [28] and a lower volatility than TTIP [29,30]. TTIP is therefore a more suitable precursor for (PE)-CVD processes at low temperature. Moreover, PECVD at atmospheric pressure offers many advantages such as to avoid the use of wet and hazardous chemical processes, the high vacuum constraints involving high cost systems and therefore the availability of implementing an in-line deposition process. Regarding the atmospheric pressure TTIP deposition, several studies focused on the TiO2 thin film deposition where the TTIP precursor is injected directly in the plasma discharge [31–32]. Only few authors concentrated their work on the deposition of TTIP in the post-discharge [33–34] and most of those were done at low pressure with and without substrate heating [35–38]. Microwave discharges in H2, O2 and H2–O2 at low pressure were used to study the effect of H and OH radicals on the deposition rates of TTIP [35]. The substrate temperature was also modified from R.T. to 300 °C and showed that increasing the temperature led to a decrease of the OH groups in the film detected by Infrared spectroscopy [35]. Another study investigated the gas-phase reaction mechanism of the remote-PECVD in a N2–O2 atmosphere where the substrate was heated from 200 °C to 350 °C [36]. They showed that the dissociation of C–H bond from TTIP molecule

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was promoted in O2 and that some anatase structures were observed for films synthesized at temperatures higher than 280 °C. Borras et al. [38] presented a systematic study of the PECVD of TTIP in a microwaveelectron cyclotron resonance reactor with a remote configuration in O2 and Ar–O2. They showed that microstructures and optical properties of the TiO2 thin films were tightly related. Heating at temperatures higher than 230 °C during the plasma treatment was required to generate anatase structures. Although some of those studies highlighted the necessity of heating the substrate to induce crystallization in the TiO2 coatings, its influence on the photodegradation is not discussed [33–34,39]. Moreover, the TTIP is often introduced through liquid droplets by evaporator or aerosol system and not in a vapour phase [40–42]. The introduction of TTIP through the vapour phase approach allows a more homogeneous introduction as well as a better control of the amount of the injected precursor. Indeed, controlling the temperature and the gas flow rate leads to the injection of very small precursor amount which might prevent any possible undesirable reaction with water vapour. However, the synthesis of titanium oxide thin films in the post-discharge is of great interest, especially for fragile samples such as polymeric substrates. Indeed, such materials being sensitive to heating, the post-discharge presents a considerable advantage compared to traditional chemical vapour deposition methods where the substrate needs to be heated to generate photoactive surfaces. In the present study, thin films of TiO2 are deposited using vapours from titanium tetraisopropoxide (TTIP) injected in the post-discharge of a RF atmospheric pressure plasma torch supplied with argon and oxygen. The synthesis is done at room temperature without any external substrate heating during the plasma deposition. Film composition and crystallinity are analysed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, respectively. The thickness and the morphology of the coatings are characterized by scanning electron microscopy (SEM). Finally, the photocatalysis and the photo-induced hydrophilicity are evaluated from the analysis of methylene blue degradation by UV– visible spectrometry and water contact angle measurements (WCA). Post-annealing treatments are realized in order to highlight changes in the TiO2 crystallinity as well as the influence on the photocatalysis properties. 2. Experimental section 2.1. Deposition of TiO2 films Liquid titanium tetraisopropoxide TTIP (Acros Organics, 98 +%) precursor was used without any further purification. Silicon Wafers (Compart Technologies) were used as substrates. For some experiments, 304 2R stainless steel was also employed to compare the influence of the substrate on the photocatalytic effect. Substrates were washed twice in an ultrasonic bath containing ethanol and acetone, respectively. The coatings were deposited with a RF atmospheric plasma torch, the Atomflo™ 250D plasma source from Surfx Technologies, operating at a frequency of 27.12 MHz and supplied with argon as carrier gas and oxygen as reactive gas. An argon/oxygen mixture was used to initiate the plasma. The vapours of TTIP were then introduced into the post-discharge by means of an argon line connected to a bubbler containing liquid TTIP heated inside a water bath at 30 °C. The TTIP/Ar flow rate passes then through a homemade diffusor placed just below the plasma torch. The diffusor consists of a circular aluminium pipe with ten holes to leave the diffusion of the argon flow rate containing precursor vapours in the postdischarge. A schematic view of the setup is depicted in Fig. 1. The deposition conditions for the coatings reported in this paper were the following ones: 70 W, O2 flow rate 11 mL/min, Ar flow rate 30 L/min, TTIP/Ar flow rate 8 L/min, TTIP temperature 30 °C, 10 min of deposition time and a gap (distance between the sample and the

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torch) of 7 mm. A thermocouple was set on the substrate during the plasma treatment to measure its temperature. A post-annealing treatment in air ambient for 2 h was conducted to improve the crystallinity of the TiO2 films at an annealing temperature of 450 °C. 2.2. Characterization 2.2.1. Water contact angles (WCA) A drop shape analyser (Krüss DSA 100) was employed to perform water contact angle (WCA) measurements. 3 μL droplets of milli-Q water were deposited onto the coatings with an automated syringe. WCAs were measured in a static mode through the tangent 1 method from the drop shape analysis software provided by Krüss. The water contact angle values reported in this paper are the average obtained from 10 different positions on the whole surface. 2.2.2. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) analyses were performed on a Physical Electronics PHI-5600 instrument. Survey scans were used to determine the chemical composition of the sample surface and narrow-region spectra, for the chemical study of the Ti 2p and O 1s peaks. Spectra were acquired using the Mg anode (1253.6 eV) with spectral width of 0.7 eV, operating at 300 W. Survey spectra were recorded at a 93.9 V pass-energy, with 5 scans accumulation (time/step: 50 ms, eV/step: 0.8 eV), at a pressure inside the analysis chamber of 1 × 10−7 Pa and high resolution spectra of the Ti 2p and O 1s peaks, at 23.5 eV pass-energy with an accumulation of 10 scans (time/step: 50 ms, eV/step: 0.05 eV). The elemental composition was calculated after the removal of a Shirley background and using the sensitivity coefficients coming from the manufacturer's handbook: SO1s = 0.63, SC1s = 0.205, STi2p3/2 = 1.1. In order to evaluate the surface contamination and the composition of the bulk of the film, sputter depth profiles using Ar+ ions were acquired. The pressure inside the ultra-high vacuum chamber was 10−5 Pa. 2.2.3. Scanning Electron Microscopy (SEM) SEM measurements have been performed in order to have information about the morphology of the film and its thickness. The equipment was a Jeol JSM-6400 with a tungsten cathode. Secondary electrons were detected by a scintillator coupled with a photomultiplier. The pressure inside the equipment was 10−4 Pa. Agglomerate size distributions are determined with the ImageJ software from 14 ∗ 11 μm2 images performed with a magnification of 8000. The analysis was limited to agglomerate sizes higher than 35 nm in diameter in order to avoid possible artefacts due to the image resolution. 2.2.4. Raman spectroscopy The LabRAM HR Evolution is a fully integrated confocal Raman microscope equipped with a high stability confocal microscope with XYZ

Fig. 1. Experimental setup of the plasma torch.

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motorized stage. The objective used was the 100× (WD = 10.6 mm) and there was a multichannel air cooled CCD detector (spectral resolution b1 cm−1, lateral resolution 0.5 μm, axial resolution 2 μm). A solid state laser (Nd — YAG, 532 nm) is mounted on the instrument. 2.2.5. Photocatalysis measurements The process consisted in exposing a sample to UV irradiation in the presence of methylene blue. The homemade UV lamp was composed of five Hg lamps (5 × 9 W) emitting at 254 nm. The TiO2 sample was placed into a glass Petri dish (without top) at a distance of 5.5 cm under the UV lamp. 10 mL of methylene blue 10−5 mol L−1 was then added to the Petri dish. The degradation of methylene blue was measured with a UV/Vis Perkin Elmer Lambda 3 double beam spectrophotometer after different time of UV exposition. The concentration of methylene blue was determined by the absorbance at λ = 660 nm, the molar absorption coefficient of the methylene blue being 79.51 103 L mol−1 cm−1 at λ = 660 nm [43]. 3. Results and discussion 3.1. Film composition XPS analyses have been performed in order to determine the surface composition of the deposited coatings. The XPS spectrum of the coating deposited from TTIP confirms the presence of a titanium-containing film [17,31,44]. Indeed, titanium and oxygen coming from the precursor are clearly identified in the survey spectra represented in Fig. 2(a). However, similarly to other studies, a percentage of carbon N20% has been detected at the surface [31,44]. Its presence can be explained either by residual amount from the TTIP precursor or by adsorption of species from the ambient air atmosphere after the deposition. In order to denote the origin of this element and to investigate its presence in the coating, a sputtering of the film by Ar+ has been performed. As shown in Fig. 2(b), no carbon is detected inside the film confirming that the carbon is just a surface contaminant which is assumed to come from the surrounding environment. The high resolution spectrum of Ti 2p is particularly interesting because of its ability to distinguish binding energies based on the chemical surrounding and therefore characterize the presence of TiO2 or Ti metallic [31,44]. Fig. 3(a) represents the Ti 2p spectrum which consists of a doublet characteristic of 2p 3/2 and 2p 1/2 components located at binding energies of 458.5 eV and 464.2 eV, respectively. These positions and the energy difference Δ of 5.7 eV are characteristics of titanium dioxide

Fig. 2. XPS spectra of a TiO2 film on steel obtained from TTIP deposition in the postdischarge of the plasma torch at atmospheric pressure (a) before and (b) after 2 min of Ar+ sputtering (70 W, 8 L/min TTIP at 30 °C,11 mL/min O2, 30 L/min Ar).

chemistry [45]. Indeed, the presence of metallic titanium would have induced a component at 453.8 eV and an energy difference Δ between 2p 3/2 and 2p 1/2 spin orbit components of 6.15 eV. Moreover, the existence of a bond between O and Ti is also evidenced in the O 1s HR spectrum depicted in Fig. 3(b). The component at a binding energy of about 530 eV is assigned to the oxygen in the TiO2 structure. The additional component at 331.8 eV is characteristics of hydroxyl groups [17]. The presence of this second oxygen component might explain the non-stoichiometric synthesized TiO2. Indeed, the O/Ti ratio obtained from the survey is higher than 2 indicating a higher concentration in oxygen compared to the stoichiometric TiO2. 3.2. Crystallinity Raman spectroscopy gives information on the sample crystallinity (Fig. 4). Similarly to other studies focusing on the deposition of TiO2 by plasma enhanced chemical vapour deposition at room temperature, the coating deposited by the plasma torch is amorphous [46–48]. Indeed, only peaks associated to silicon substrate (519.9 cm−1 and 302.0 cm−1) are observed in the as-deposited TiO2 film [49–50]. After calcination at 450 °C, three Raman bands at 144, 393 and 635 cm− 1 corresponding to the Eg, B1g and Eg modes for the anatase structure of TiO2 were observed [51–52]. None of the bands associated to rutile structure were observed in the Raman spectra. This is in agreement with literature since no rutile TiO2 was observed to be formed in that temperature range [53]. A deposition at high temperature, usually N450 °C, or an annealing process post-deposition are therefore required to (partially) crystallize the TiO2 coating [46–48].

Fig. 3. High resolution spectra of (a) Ti 2p and (b) O 1s of TiO2 film deposited on steel.

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Fig. 4. Raman spectra of (a) reference anatase, (b) Si wafer native substrate, (c) asdeposited TiO2 coating on Si wafer and (d) annealed TiO2 coating on Si wafer (450 °C for 2 h). (10 min, 8 L/min TTIP at 30 °C, 70 W, 11 mL/min O2, 30 L/min Ar).

Some X-ray diffraction (XRD) measurements were done but they are not presented in this paper. Indeed, XRD is less sensitive than Raman and did not highlight the presence of a partial crystallinity. 3.3. Morphology and thickness Scanning electron microscopy has been used in order to analyse the morphology and the thickness of the deposited coatings. The presence of a TiO2 coating at the surface of the silicon wafer is clearly noticeable in the FEG-SEM cross sections in Fig. 5. The contrast between the deposited film (in white) and the substrate (in dark

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grey) allows observing the morphology of the coating. The films do not appear to be very compact whatever the deposition time and according to Fig. 5, the coating exhibits a structure made of two parts: a continuous film at the substrate interface terminated by agglomerates. The agglomerates of TiO2 particles are also clearly visible at the surface and seem to be rather homogeneously dispersed on the whole surface whatever the deposition time as seen in Fig. 5(a–c). However, the size of those aggregates varying between ten nanometres and few micrometres is influenced by the deposition time. Using the ImageJ software, we have estimated the size distribution of the agglomerates from the SEM images in Fig. 5(a–c) as illustrated by the histograms. Agglomerates with diameters larger or equal to 100 nm represent about 5% of the total number of agglomerates for a deposition time of 2 min. For a deposition time of 5 min and 10 min, these agglomerates represent approximately 26% and 61%, respectively. Such a phenomenon was observed in other studies and the agglomerates' formation is described as a parallel process to the thin film growth [25,41]. The deposition rates have been estimated based on the thicknesses measured from the cross-section SEM images. The considered thicknesses do not take in account the agglomerates as shown in Fig. 5(d–f). It appears that the deposition rate is not constant (i.e. approximately 11.5 nm/min, 13 nm/min and 63 nm/min for deposition times of 2 min, 5 min and 10 min, respectively). This modification in the growth mechanism is not well understood but could be induced by variations in substrate temperature as a function of the deposition time (see Fig. 6). Indeed, as seen in Fig. 6, the substrate temperature increases over time (i.e. 65 ± 5 °C at 2 min, 85 ± 3 °C at 5 min and 100 ± 5 °C at 10 min). This measurement was done for the two different substrates and gives the similar results for the stainless steel and the silicon wafers. 3.4. Wettability The hydrophilic character is clearly enhanced by the presence of a TiO2 film since the native silicon substrate and the plasma treatedsilicon substrate have a WCA of approximately 32° and 22°, respectively,

Fig. 5. SEM images with insets showing histograms of particles size distribution, and cross-section FEG-SEM images of TiO2 deposited onto a Si wafer (8 L/min TTIP at 30 °C, 70 W, 11 mL/min O 2 , 30 L/min Ar). 8000 magnification surface images (a) 2 min (b) 5 min (c) 10 min. Cross-section images (d) × 25000–2 min deposition, (e) × 15000–5 min deposition, (f) × 15000–10 min deposition.

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Fig. 6. Substrate temperature (silicon and stainless steel) as a function of the treatment time (8 L/min TTIP at 30 °C, 70 W, 11 mL/min O2, 30 L/min Ar).

while the presence of TiO2 film induces a full spreading of the water droplet at the surface (Fig. 7). According to the literature, only a few studies have shown as high wettability with WCA lower than 10° [31, 54]. Although the UV irradiation of deposited TiO2 coatings induces a super-hydrophilicity, the WCA recorded on the as-deposited surfaces is usually higher than 30° [26,47]. The water contact angle is relatively constant as a function of the deposition time since all the surfaces are super-hydrophilic for a deposition time higher than 2 min as seen in Fig. 8. Although the wettability of coatings deposited from higher deposition times is better preserved with time, the hydrophobic recovery in the dark room described in the literature appears and WCAs of 60–70° are reached whatever the deposition conditions [17,31]. Looking at the SEM measurement, the roughness created by the agglomerates could be responsible for the low water contact angles observed since water can be absorbed into these cavities. Moreover, it is well known that, according to the Wenzel model, increasing the roughness of an already hydrophilic surface will enhance the wettability character [55]. However, adsorbing gaseous contaminants from the air can alter the surface which changes the surface wettability from hydrophilic to hydrophobic.

3.5. Photocatalytic activity and photo-induced hydrophilicity TiO2 is known to display specific photocatalytic properties when exposed to UV-light [11,15–17]. The first property is the photo-

Fig. 8. WCA ageing study of the TiO2 films for different deposition times (8 L/min TTIP at 30 °C, 70 W, 11 mL/min O2, 30 L/min Ar).

induced decomposition of organic compounds and the second one is the photo-induced hydrophilicity of the surface. TiO2 being a semiconductor, UV light can excite electrons and holes which lead to the formation of superoxide radical anions (•O− 2 ) and hydroxyl radicals (OH) while in contact with water and molecular oxygen. These two reactive species will then decompose organic compounds at the TiO2 surface [56]. In this study, the photocatalytic property is evaluated through the degradation of the very well-known methylene blue (MB) [14,43,57]. In order to have a reference, the methylene blue has been exposed to UV irradiation during 90 min with and without native substrates. The concentration of MB is modified by UV and is characterized by a degradation of approximately 20% after 90 min without any substrate. If a substrate is immersed into the MB solution, the MB concentration, while exposed to UV, might be influenced by the nature of the substrate. Indeed, as seen in Fig. 9(a), the concentration of the MB in the solution decreases because the MB can be adsorbed on the rough steel surface [58]. Silicon wafer substrates being smoother than steel, no significant variation of the MB concentration in the solution is observed. The MB degradation in the presence of a TiO2 coating is clearly increased as seen in Fig. 9(a) and summarized in Table 1. The TiO2/Steel surface shows an increase of the degradation compared to the bare steel substrate. The complex slope of the curve could be due to a two step process. At the beginning, the adsorption of MB, together with the UV photodegradation lead to a fast decrease of the remaining MB in solution; when the full surface is covered with adsorbed MB, the slope decreases, the degradation being only driven by UV photodegradation. However, the TiO2 deposited onto the silicon wafer

Fig. 7. WCA images of (a) native Si wafer substrate WCA = 32°, (b) Si wafer substrate with plasma activation WCA = 22° and (10 min, 70 W, 11 mL/min O2, 30 L/min Ar) (c) Si wafer substrate with TiO2 film after plasma activation WCA b 10° (10 min, 8 L/min TTIP at 30 °C, 70 W, 11 mL/min O2, 30 L/min Ar).

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Fig. 10. Evolution of the static WCA of 60 days aged TiO2 films after UV irradiation (8 L/min TTIP at 30 °C, 70 W, 11 mL/min O2, 30 L/min Ar, 2 min).

Fig. 9. (a) Evolution of the methylene blue concentration as a function of the UV irradiation time. MB reference (black curve), substrates without TiO2 film (dashed curves), and substrates with TiO2 film (full curves). (b) Recycling property of the annealed TiO2 coatings.

substrate leads to a much higher degradation compared to TiO2 deposited on steel. In the literature, some authors attempted to explain the positive impact of Si on the photocatalytic effect of TiO2 by studying TiO2/SiO2 films [59–60]. They showed that a chemical interaction occurred between Ti and Si and a Ti–O–Si bond is created and enhances the ability of TiO2 to photodegrade pollutants. Although in the present study, the presence of a Ti–O–Si chemical bond has not been experimentally evidenced, the enhanced catalytic effect observed is consistent with this assumption. As shown in Fig. 9(a), both the as-deposited (amorphous) and the annealed (anatase structure) TiO2 coatings are photocatalytic. The anatase crystalline structure is known to induce a higher photocatalytic activity than amorphous films (N 80%) [61]. This statement is in agreement with our observation since about 85% of the MB is degraded in presence of annealed TiO2 coatings, compared to 60% with the amorphous TiO2 coatings. However, the photo-activity induced by the amorphous coatings is not negligible as also shown in other studies of titanium-based precursors PECVD [54]. For instance, Sobczyk-Guzenda et al. studying the capability of killing bacteria showed that RF-PECVD of TiCl4 at low pressure led to amorphous coatings with a 20% of killed bacteria while the annealed crystalline coatings induced 90% [54]. The

recycling efficiency of both the as-deposited and the annealed TiO2 coatings has also been investigated through the immersion of the same samples in new MB solutions after each photocatalytic measurement. Only the annealed TiO2 results are presented in this article but similar results were observed for the as-deposited coatings. As shown in Fig. 9(b), the photocatalytic activity is constant after at least 3 cycles of 90 min UV illumination indicating a very good stability and photocatalytic efficiency of the coatings over time. This recycling property was already analysed by other authors [62–64] and proved the good adhesion of the coating on the substrate because when fresh MB solutions are used, the coating still presents the same activity. The photo-induced hydrophilicity is analysed through the WCA measurements as a function of the UV irradiation time (see Fig. 10). It is shown that the WCA is drastically reduced and drops to lower values than 10° within 20 min of UV irradiation, indicating the recovery of the super-hydrophilicity. The change in the wettability of TiO2 films during UV irradiation is assumed to be due to the increase of hydroxyl groups on the surface [15–17]. It is suggested that this increase of OH groups is caused by dissociative adsorption of water in vacancies. Thanks to the semiconducting properties of TiO2, holes are photo-generated and are trapped at lattice oxygen sites at the surface of the TiO2 film. The bond between the titanium atom and the lattice oxygen is then weakened and can be broken to react with water from ambient air. This reaction will therefore “heal” the TiO2 surface by producing surface hydroxyl groups which tend to make the surface hydrophilic [17]. 4. Conclusion TiO2 coatings were deposited onto different substrates using TTIP injected in the post-discharge of a plasma torch at atmospheric pressure. These coatings are characterized by agglomerates at the upper surface with sizes influenced by the deposition time. Without any post-thermal treatment, the coatings are amorphous while anatase TiO2 structure is observed by Raman spectroscopy after annealing at 450 °C. The crystallization of the coating induces an improvement of the photo-catalytic activity. It is shown that upon UV-light, the

Table 1 Summary of methylene blue degradations after 90 min.

% MB degradation

No substrate

Silicon wafer No coating

As-deposited TiO2

Annealed TiO2

No coating

Stainless steel As-deposited TiO2

21%

21%

61%

85%

35%

39%

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