Preparation of nanocrystalline titania thin films by using supercritical carbon dioxide modified by water. Prague 6, Czech Republic

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1 Preparation of nanocrystalline titania thin films by using supercritical carbon dioxide modified by water Marie Cerhová 1, *, Marie Sajfrtová 1, Lenka Matějová 2, Vladislav Dřínek 1, Stanislav Daniš 3, Věra Jandová 1 1 Institute of Chemical Process Fundamentals of the ACS, v.v.i., Rozvojová 135, Prague 6, Czech Republic 2 Institute of Environmental Technology, VŠB-Technical University of Ostrava, 17.listopadu 15/2172, Ostrava Poruba, Czech Republic 3 Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Prague 2, Czech Republic *Corresponding author: cerhova@icpf.cas.cz, Tel.: The supercritical fluid crystallization by pure and water modified supercritical carbon dioxide was used for the preparation of crystalline and pure nanostructured TiO 2 thin films. Simultaneous, organic precursors used in the synthesis of solution to prepare thin films by solgel method were removed. The influence of temperature and the volume of solvent on the microstructure and the purity of TiO 2 thin films was monitored by Raman spectroscopy and X-ray diffraction analysis. Anatase crystalline structure was achieved by the process with scco 2 modified by 30 wt. % water at temperature 150 C. INTRODUCTION Titania films are materials of great interest for various applications such as the dyesensitized electrodes used in solar energy conversion [1] or gas sensors [2]. TiO 2 forms three crystalline phases (rutile, anatase, brookite), of which anatase is most investigated because of its high photocatalytic activity. Due to its chemical stability, strong oxidation activity and non-toxicity, anatase can be used in environmental applications, such as self-cleaning surfaces and coatings [3] air- and water-purification [4] or for the decomposition of organic compounds [5]. The use and performance of TiO 2 for a given application depends not only on the phase composition, but also on hydrophilicity, specific surface area, crystallinity and crystallite-size. These properties can be significantly influenced by the preparation method used. The most commonly used method for transforming amorphous titania to anatase is calcination (thermal treatment, TT) [6]. Nevertheless, this simple approach has some disadvantages, such as recrystallization, excessive sintration, crystallite growth and a decrease in the specific surface area. Furthermore, the properties or photocatalytic performance cannot be easily managed. Therefore, it is important to develop a method by which the structural, textural and physicochemical properties can be better controlled. Current attention is focused on the preparation of nanostructured TiO 2 particles using by supercritical carbon dioxide drying combined with thermal processing. It was found that the pretreatment using supercritical carbon dioxide (scco 2 ) proposed for lowering the process temperature of sol-gel derived metal oxide film helps to increase the thermal stability and photocatalytic activity of films [7]. Matějová et al. utilized to obtain the nanocrystalline TiO 2 thin films supercritical carbon dioxide drying with the addition of water. They demonstrated that the modification of scco 2 small amount water causes the direct crystallization of pure titania with high specific surface area [8]. The advantage of this unique combination of solvents consists not only in their low price and availability, but mainly in their chemical universality to dissolve broad range of compounds showing different polarity.

2 Based on this knowledge, in this study we tested a technique that used a combination of supercritical CO 2 and water for the synthesis of crystalline and pure nanostructured TiO 2 without any subsequent thermal processing. The process with pure and modified scco 2 by 30 wt. % of water was tested. The effect of extraction temperature ( C) and the volume ( g) of CO 2 passed through the extractor on microstructure and purity of TiO 2 thin films were examined. The prepared thin films were characterized with respect to the (micro)structural properties by Raman spectroscopy. The most promising thin films were analyzed by means of X-ray diffraction to determine the crystallite size and the phase composition. MATERIALS AND METHODS Materials Cyclohexane (p.a., Lachner, Neratovice, CR), Triton X-114 ((1,1,3,3,- Tetramethylbutyl)phenyl-polyethylene glycol, Sigma-Aldrich, USA), Ti(OCH 2 (CH 3 ) 2 ) 4 (Titanium(IV) isopropoxide, Sigma-Aldrich, USA) and distilled water were used for the preparation of precursor solution of TiO 2 films. Carbon dioxide (> 99.9 %) for SFE was purchased from Linde Technoplyn (Prague, CR). Sol preparation and deposition of thin films Thin films of titanium dioxide were prepared by applying a sol on the soda-lime glass by dip-coating. The solution was obtained by sol-gel techniques controlled within reverse micelles of a nonionic surfactant Triton X-114 in cyclohexane. The molar composition of the titanium solution was as follows: cyclohexane : Triton X-114 : water : Ti(OCH 2 (CH 3 ) 2 ) 4 = 11: 1:1:1. Cyclohexane, Triton X-114 and water was stirred for 15 minutes until nezhomogenizovala, and not formed with reverse micelles. Then, to the mixture was added titanium isopropoxide and stirring was continued for further 20 minutes. Supercritical fluid crystallization (SFC) The glass with thin films was placed in a steel holder and placed into the extraction column (150 ml, inner diameter 30 mm) filled with glass beads serving as the solvent flow distributor. The experiments were performed in the apparatus Spe-ed SFE (Applied Separations, USA). Carbon dioxide was sucked from a pressure container using a highpressure pump cooled by water to 5 C. Water for the modification of scco 2 was supplied at a constant flow rate by the high pressure pump LCP (ECOM s.r.o.) and mixed with scco 2 before entering the column to reach concentration in scco 2 30 wt. %. The solution flowing from its upper end was expanded to the ambient pressure in a heated micrometer valve and the extract was collected in empty glass vial at the ambient temperature. Flow rate and the amount of scco 2 passed through the extraction column were measured by the gas meter at the outlet from the separator. The processing of thin films crystallization was performed under a constant pressure of 30 MPa and temperatures of 40 and 150 C by pure or water modified scco 2. The volume of scco 2 passed through the extraction column was varied between g. To improve the purity of TiO 2 thin films the treatment by pure scco 2 at lower temperature of 40 C before the crystallization by water modified scco 2 was tested. SFC conditions are given in Table 1.

3 Table 1. Experimental design and conditions of supercritical fluid crystallization. Experiment Temperature, C/solvent (volume of CO 2, g) Nr. 1 st step 2 nd step 1 40 CO 2 (100) CO 2 (100) 3 40 CO 2 +W (100) 4 40 CO 2 +W (200) CO 2 +W (50) CO 2 +W (100) CO 2 +W (200) 8 40 CO 2 (100) 150 CO 2 +W (100) 9 40 CO 2 (200) 150 CO 2 +W (100) Characterization of thin films The presence of anatase crystalline structure and the purity of TiO 2 thin films was monitored by Raman spectroscopy. The phase composition and crystal size of TiO 2 thin films were determined by X-ray diffraction analysis (XRD). RESULTS The character and purity of TiO 2 thin films Raman spectra of TiO 2 thin films processed by SFC under different conditions are compared in Figure 1.Signal intensity in different areas of Raman shift indicates the occurrence of undissolved surfactant and other organic impurities. In the gray field area the anatase peaks are available. Figure 1: Raman spectra of TiO 2 thin films processed by a various conditions of SFC (see Tab. 1) : Valence and deformation vibrations: (=C H, 3074 cm -1 ), ( C H, 2938 cm - 1 ), (C=C, 1613 cm -1 ), (H C H, 1465 cm -1 ) a (C C aliphatic, 1292 cm -1 ), region of anatase peaks in gray field area: cm -1.

4 Thin films prepared with pure scco 2 (Exp. 1, 2) were amorphous as expected. Moreover, binding of a surfactant, which are characterized by Raman spectra and deformed valence bonds (=C H, 3074 cm -1 ), ( C H, 2938 cm -1 ), (C=C, 1613 cm -1 ), (H C H, 1465 cm -1 ) a (C C aliphatic, 1292 cm -1 ) indicated that the solubility of a surfactant and other organic substances resulting in the production of the sol is insufficient. When water modified scco 2 at 40 C (Exp. 3, 4) was used the impact of the volume of water affecting the thin films was proved. The processing by SFC with smaller amount of water (Exp. 5) was for crystallization of TiO 2 thin film insufficient. The crystallization of TiO 2 thin film was achieved by the treatment with 100 g of water modified scco 2 at 150 C (Exp. 6) as is evident from the spectra from belts 168, 418, 532 and 653 cm -1, that indicate the structure of anatase. Moreover, the solubility of the surfactant significantly increased. Larger amounts of solvent at 150 C caused unwanted removal of the film from the soda-lime glass substrate (Exp. 7). The anatase peaks were also observed when experiment at the optimal conditions and with treatment by pure CO 2 before the crystallization by water modified scco 2 (Exp. 8, 9) was performed. However, the increased background noise of Raman spectra indicated the partial dissolution of the film in high dense CO 2. The structure and crystal size XRD analysis of all prepared titania thin films confirmed the presence of anatase crystal structure as evident from the typical bands of the anatase structure in Figure 2. Figure 2: XRD spectra of TiO 2 thin films processed by SFC with water modified scco 2 at 150 C (6), pure scco 2 (100 g) at 40 C followed by water modified scco 2 at 150 C (8), pure scco 2 (200 g) at 40 C followed by water modified scco 2 at 150 C (9). A anatase.

5 The presence of the crystalline structure on TiO 2 thin films observed in Raman spectra was confirmed by XRD analysis for all tested films, as evident from considerable strips anatase structure throughout the measured range. The particle size data of nanocrystalline TiO 2 obtained by XRD analysis play an important role in the physico-chemical behavior of the material in terms of chemical and phase stability and chemical reactivity. The comparison of anatase crystallite-sizes determined for TiO 2 thin films prepared by water modified scco 2 under different experimental design and conditions is summarized in Fig. 3. Figure 3: TiO 2 anatase crystallite-size of thin films processed by SFC with water modified scco 2 at 150 C (6), pure scco 2 (100 g) at 40 C followed by water modified scco 2 at 150 C (8), pure scco 2 (200 g) at 40 C followed by water modified scco 2 at 150 C (9). A anatase. As is apparent from Figure 3, the size of anatase crystallites varied depending on the process conditions and the solvent volume in the range of 2.0 to 6.3 nm. The largest crystallites were achieved during the one step processing by water modified scco 2 at 150 C after consumption of 100 g of CO 2. When TiO 2 thin films were treated by pure scco 2 before the crystallization by water modified scco 2 the crystal size decreased. Increasing volume of scco 2 resulted in higher decline of the crystal size. Thanks to the higher density of scco 2 the important components of thin films were probably dissolve. CONCLUSION Using the supercritical fluid crystallization by water modified supercritical carbon dioxide nanocrystalline TiO 2 thin films without any further heat treatment were successfully obtained. SFC by pure scco 2 and by water modified scco 2 at 40 C resulted in the amorphous TiO 2 thin films. The increasing amount of scco 2 negatively affect the crystal growth. At the optimal conditions of SFC by water modified scco 2 (30 MPa, 150 C, 100 g of scco 2 ) the anatase crystals with the size of 6.3 nm were obtained.

6 Acknowledgement The financial support of the Grant Agency of the Czech Republic (project reg. no S) is gratefully acknowledged. REFERENCES: [1] XU, J., ZHANG, B., XIAO, L., LIU, X., ZHANG, Y., YAO, J., DAI, S., PAN, X., Journal of Alloys and Compounds, Vol. 626, 2015, p [2] LI, Y., WLODARSKI, W., GALATSIS, K., MOSLIH, S. H., COLE, J., RUSSO, S., ROCKELMANN, N., Sensors and Actuators B, Vol. 83, 2002, p [3] FUJISHIMA, A., RAO, T. N., TRYK, D. A., Electrochimica Acta, Vol. 45, 2000, p [4] SÖKMEN, M., ÖZKAN, A., Journal of Photochemistry and Photobiology A: Chemistry Vol. 147, 2002, p. 77. [5] ANDERSSON, M., ÖSTERLUND, L., LJUNGSTRÖM, S., PALMQVIST, A., Journal of Physical Chemistry B, Vol. 106, 2002, p [6] HSIEN, Y.-H., CHANG, C.-F., CHEN, Y.-H., CHENG, S., Applied Catalysis B: Environmental, Vol. 31, 2001, p [7] WEI, M., WANG, K., YANAGIDA, M., SUGIHARA, H., MORRIS, M. A., HOLMES, J. D., ZHOU, H., J. Mater. Chem., Vol. 17, 2007, p [8] MATĚJOVÁ, L., SAJFRTOVÁ, M., MATĚJ, Z., FAJGAR, R., EMSF 2014, Marseille, France, p.161 (Full text on Flash disc).