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Accepted Manuscript Hydrophilic/hydrophobic and optical properties of B2O3 doped TiO2 sol-gel thin films: Effect of B2O3 content, film thickness and surface roughness Wassila Saidi, Nasreddine Hfayedh, Adel Megriche, Mihaela Girtan, M. El Maaoui PII:

S0254-0584(18)30250-5

DOI:

10.1016/j.matchemphys.2018.03.080

Reference:

MAC 20483

To appear in:

Materials Chemistry and Physics

Received Date: 14 December 2016 Revised Date:

6 March 2018

Accepted Date: 27 March 2018

Please cite this article as: W. Saidi, N. Hfayedh, A. Megriche, M. Girtan, M. El Maaoui, Hydrophilic/ hydrophobic and optical properties of B2O3 doped TiO2 sol-gel thin films: Effect of B2O3 content, film thickness and surface roughness, Materials Chemistry and Physics (2018), doi: 10.1016/ j.matchemphys.2018.03.080. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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University Tunis El Manar

Department of Chemistry

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Faculty of Sciences

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Laboratory of Applied Mineral Chemistry

Hydrophilic/Hydrophobic and Optical Properties of B2O3 Doped TiO2

Wassila Saidi

a*

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Sol-Gel Thin Films: Effect of B2O3 Content, Film Thickness and Surface Roughness.

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Corresponding author

Email: [email protected] Authors:

a a b a Nasreddine Hfayedh , Adel Megriche , Mihaela Girtan , M. El Maaoui

a

Université de Tunis El Manar, Faculté des sciences de Tunis, (UR11ES18) Laboratoire de

chimie minérale appliquée b

Université d’Angers, laboratoire photonique (LPHIA), LUNAM - 2 Bd, Lavoisier, 49045,

Angers, France.

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Hydrophilic/Hydrophobic and Optical Properties of B2O3 Doped TiO2 Sol-Gel Thin Films: Effect of B2O3 Content, Film Thickness and Surface Roughness.

a

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a* a a b a Wassila Saidi , Nasreddine Hfayedh , Adel Megriche , Mihaela Girtan , M. El Maaoui

Université de Tunis El Manar, Faculté des sciences de Tunis, (UR11ES18) Laboratoire de

chimie minérale appliquée

Université d’Angers, laboratoire photonique (LPHIA), LUNAM - 2 Bd, Lavoisier, 49045,

SC

b

M AN U

Angers, France.

Abstract

High transparent B2O3-doped TiO2 thin films were deposited on glass substrates using sol-gel method and spin coating. B2O3 content was varied from 0 to 20 weight %. Each film layer was calcined at 500 °C for one hour. Film thickness was determined by profilometer and UV techniques. Monolayer thicknesses decrease from 55 nm to 47

nm when B2O3 content

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increases. For multilayer films, the thickness grows by steps of about 50 nm by layer and slightly decreases when boron content increases. The presence of B2O3 affected also roughness, light transmission and optical gap. As an example, for two layer films, Roughness decreases from 1.1 to 0.65 nm when B2O3 increases from 0 to 20 %. Light transmission at

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λ= 500 nm and optical gap were enhanced from 93 to 97 % and from 3.59 to 3.72 eV respectively. All these changes could be explained by a better melting of the thin layer at 500

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°C, This was confirmed with XR diffraction and AFM analysis. The second part of this study concerned the variation of hydrophobic, hydrophilic properties. Water contact angle was measured in standard conditions. It was found for single layer samples, that θ increased from 16 to 61 ±1° when B2O3 content varies from 0 to 20 %. The study was performed also for samples having 2, 3 and 4 layers. Contact angle increased slowly with film thickness. The highest angle was equal to 75 ° (± 1°); it was obtained for samples having four layers and doped with 20 % B2O3. The dependence of θ with physical properties particularly with surface roughness and thickness was also examined.

ACCEPTED MANUSCRIPT Keywords: TiO2 thin films, B2O3 doped TiO2 thin films, hydrophobic properties, and hydrophilic properties.

1. Introduction

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Hydrophobic or hydrophilic properties are very interesting topics because of their numerous applications in daily life. The most desired materials are either superhydrophilic or superhydrophobic. In the system Water- material- air, the former have a contact angle θ near to zero, in this case water spread on the solid surface and

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covers it totally [1]. Such materials are interesting because fog transforms in thin transparent water films improving better visibility. They are also necessary to achieve

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catalytic reactions where the contact must be optimal. The second materials are hydrophobic; contact angle is superior to 90° [2]. They are requested because they are self cleaning; water rolls in spherical drops and clean out dust and bacterials matter which could stick to lotus leaves, bird feathers or skin animals.

Pure TiO2 thin films were extensively investigated. They were made first to study the decomposition of water in hydrogen and oxygen [3].

However, the major part of

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research concerns catalytic properties. Due to its energy gap superior to 3.2 eV, TiO2 films have a high oxidative power when they are submitted to UV radiations; the latter make also TiO2 films superhydrophilic enhancing also the contact between the catalyst

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and water solutions. This allows very important applications mainly into the environmental field. Organic wastes are easily cleaned as well as many toxic impurities

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like cyanide ions [4, 5].

In this paper, we were interested by Boron doped TiO2 films. Our objective was to study their hydrophilic properties and to measure their transparency and gap value. Literature shows that these properties are highly affected by both physical and chemical factors [6]. Physical factors are mainly the morphological aspects of TiO2 thin film surface like porosity and surface roughness. The latter was the most studied parameter. Thus, the well known lotus effect, and similar other natural materials are superhydrophobic. When water rolls on lotus leaves or on the feathers of birds, it rolls in fact on a surface composed with very small peaks, or small microsillons, large of about 50 nm each [7]. There are many models that attempt to describe the wetting of textured surfaces. Ana Bora et al [8] studied the application of the three most important

ACCEPTED MANUSCRIPT models: Wenzel model [9], Cassie-Baxter Model (C-BM) [10] and the Miwa-Hashimoto model (M-HM) [11]. According to the Wenzel model, there is a relationship between surface roughness (r) and cosθ, r is the ratio between the actual surface area of the rough material and the apparent surface area) [9]. For hydrophilic solids (θ< 90 °), hydrophilicity increases when r increases, i.e., when roughness increases. Conversely,

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for hydrophobic surfaces (θ> 90 °), surface become more and more hydrophobic when surface roughness increases. The Cassie-Baxter model concerns heterogeneous surfaces composed with two different materials. For porous materials, particularly when air remains present into the wetted surface, one of the two materials may be air [10]. In the

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third model, called Miwa-Hashimoto (M-HM) [11], the authors propose one relationship between the contact angle, the surface fraction actually in contact with water and the roughness considered as the ratio between the area of the liquid-solid

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interface and the projected surface area.

Chemical factors affecting hydrophobic properties concern chemical composition of surface films, particularly dipole moment and abundance of OH hydroxyl groups. H2O is a molecule having a high dipole moment and is easily attracted with surface hydroxyls. When the Van der Walls bonds increase, the water contact angle decreases.

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In our case, we have to consider the crystallographic nature of TiO2, rutile, anatase or amorphous phase. Agatino Di Paola et al studied the variation of the contact angle of TiO2 according to the crystal phase. The anatase phase has a contact angle of about 23 °; the rutile has a θ of about 25 °, whereas the brookite phase is the most hydrophilic (θ=

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10 °) [12]. No mention was given for the amorphous phase which is really present in many samples particularly at very low thickness.

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Thin film deposition method is also important mainly because the synthetic methods need different starting materials and different heating temperatures, this produce different surface textures and various OH content. S. H. Nam et al compared hydrophilic properties of TiO2 thin films prepared by sol–gel method and those by reactive magnetron sputtering system [13]. The former exhibited lower water contact angle (35 °) than sputtering technic in which θ= 50 ° on glass substrate [13] and 70 ° on silicon wafers (100) substrate [14]. This could be induced by the difference on the two surface roughnesses obtained by these techniques. Contact angle of TiO2 films can be also enhanced by doping with various elements. This factor may change both the chemical film composition and the surface roughness.

ACCEPTED MANUSCRIPT Dang Mau Chien et al doped TiO2 with Al2O3 [15]. They observed that roughness decreases from 7.52 nm to 2.82 nm when Al2O3 content increases from 0 to 33.3 % whereas contact angle increases from 16.7 ° for pure TiO2 film to 73 ° for TiO2 doped with 33.3 % Al2O3. This example confirms that contact angle increases when surface roughness decreases. The same results were obtained for thin TiO2/SiO2 films which

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are known to possess good hydrophilic properties [15]. Weerachai Sangchay et al studied the SnO2 effect on the hydrophilic properties of TiO2 films, they observed that the contact angle, in absence of UV irradiation, decreased from 44.64 ° to 26.13 ° when the SnO2 content increased from 0 to 1 [16].

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This work is focused on hydrophilic properties of thin and transparent TiO2 films doped with B2O3. This oxide was chosen as dopant for the expected following benefits: i)

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Boron oxide is a non-toxic material which is largely used for the production of glasses melting at low temperature and various ceramic enamels [17], ii) It is used in borosilicates glasses like “Pyrex” in a high yield, 12 % B2O3 . It enhances aqueous and chemical durability because of the concomitant decrease of Na2O content which is no more needed to melt glass, Na2O falls from 15 to 4.5 % for sodalime glass and borosilicate glass respectively [18]. We expect that the B2O3 doped TiO2 films will

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present smoother surfaces and lower roughness than pure TiO2 films. In this work, B2O3- doped TiO2 films were deposited with the sol gel method and calcined at 500 °C. A

large

range

of

film

thicknesses

was

used.

The

relationship

between

hydrophilic/hydrophobic properties of TiO2 films have been examined in terms of B2O3

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content, film thickness, surface morphology and optical properties particularly

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transparency and gap energy.

2. Experimental methods 2.1. Preparation of B2O3-doped TiO2 thin films

Pure and mixed TiO2/B2O3 thin films were prepared via sol–gel spin coating method as described in our previous work [19]. TiO2 precursor solution was prepared by mixing ethanol, acetic acid as a stabilizing agent and titanium isopropoxide. After aging for 4 h, a homogeneous yellow solution was obtained. It’s designated as A solution. B2O3 precursor solution was achieved by dissolving 20 g of boric acid (H3BO3) into 100 g of methanol. The homogeneous solution so obtained was called B solution. The dipping B2O3-doped TiO2 solution was prepared by mixing A and B solutions in various vol. % ratios. The films were

ACCEPTED MANUSCRIPT deposited on lime glass sheets by spincoating using a spinning speed and time of 3000 rpm and 1 min respectively. The samples were dried at 80 °C and calcined at 500 °C for 1 h. The film thicknesses were adjusted indirectly by repeating the deposition and calcination procedures. 2.2. Characterization methods

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The Boron-doped TiO2 films were examined by X-ray diffraction using Cu Kα radiation (1.5406 Å) Bruker D8 Advance diffractometer. The surface morphology of thin films was determined using an Auto Probe CP-Research AFM (Thermo-microscopes) with a lateral resolution of 5 nm. UV transmittance analysis was monitored by a near-infrared to ultraviolet

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spectrophotometry (NIR-UV-VIS) PerkinElmer type in the wavelength range of 200–2250 nm. The TiO2-B2O3 film thicknesses were measured by a profilometer VeecoTM Dektak 6M. The Contact Angle (CA) measurements have been performed with a Data Physics OCA 15EC

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goniometer, at room temperature and 75 % relative humidity, in a sessile drop arrangement. 0.5 µl water drop volumes have been used to avoid gravitational drop shape alteration and to diminish evaporation effects during CA measurements. 3. Results and discussion

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3.1. Structural analysis

In order to determine the influence of Boron doping and film thickness on the structure of B2O3-doped TiO2 thin film, XRD patterns were plotted in Fig.1a and Fig.1b corresponding to 3 and 4 layers respectively. The measured values of thin film thicknesses for various layers

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and different boron content were given in table 1. The increase of film thickness with the number of dippings was found to be almost equal to n single dipping thickness.

The

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crystallite sizes of the crystalline species existing into B2O3-doped TiO2 films were deduced from the peak (101) from XRD line broadening using Debye Scherer’s equation: D=

. 

ɵ

(1)

Where λ is the wavelength of the X-ray, β denotes the full width at half maximum intensity (FWHM) and θ is the Bragg’s angle. The results are shown in table 1. As shown in Fig.1, B2O3-doped TiO2 films revealed a mixture of amorphous and crystalline structures. XRD patterns of B2O3-doped TiO2 single layer were given in a previous work [19] , we observed a poor crystallization state which is manifested by the presence of a broad peak centered on the position 2θ= 25.35 ° , no peak was observed either for one or two layers.

ACCEPTED MANUSCRIPT After 3 dippings, we observe that pure TiO2 film presents a bump in which there is a narrow peak at 2θ= 25.35 ° which was attributed to the (101) plane of anatase phase [20]. This result shows the presence of a mixture of an amorphous phase and an anatase phase. With increasing the film thickness, the intensity of the narrow peak increases and that of the broad peak decreases (see Fig.1b). This could be explained by the fact that the first films were

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nanofilms and cannot diffract X rays very well. The growth of the size of TiO2 crystals became faster when TiO2 materials increase, another peak at 48.11° appeared. It is assigned to (200) plane of anatase phase [20].

We can see on table 1, the crystallite size for pure TiO2 films increases gradually from

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18.1 to 21.9 nm when the thickness increases from 170 to 227 nm (± 2 nm). This is in agreement with many results reported in literature by Mechiakh et al. and Yongjun Chen et al

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[21, 22]. Concerning the effect of boron contents, we observe that intensity of the peak at 2θ= 25.35 ° decreases when B2O3 increases. The crystallite size of TiO2 anatase decreases in the same manner. This result is very important, it indicated that boron addition promotes the transformation of anatase to amorphous phase and improves the formation of a glass phase; this is conform to the use of B2O3 as a melting agent for ceramic enamels instead of the lead contact angle.

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oxide which was used before. We will see that the amorphous phase will have an effect on the

3.2. Morphological properties

Fig.2 shows the 3D AFM images respectively of pure TiO2 and TiO2 doped with 20 % B2O3

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for different dippings number (2, 3 and 4 dipping). The Boron doped TiO2 films deposited on glass substrates were fully transparent. As shown in table 4, the RMS surface roughness is

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affected both by the boron content and the number of dipping. For example, the results for two layers show that the RMS decreases from 1.1 to 0.38 nm with increasing Boron doping from 0 % to 20 % B2O3. This result indicates that high B2O3 contents promote the formation of a flatter surface; this is caused by the better melting of TiO2 films. For given boron content, the measured roughness increases with increasing film thickness; for example for 20 % B2O3 doped TiO2 films, surface roughness increases from 0.29 to 2.14 nm when film thickness varied from 47 to 215 (± 2 nm). This is in agreement with the larger size of crystals observed on the surfaces of thicker films.

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Fig.3 shows UV-Visible transmittance spectra of TiO2 thin films, for different boron content from 0 to 20 % and different dipping number (2, 3 and 4 dippings). Spectra showed a similar shape for all samples. There begin by a strong absorption at λ