Sol gel obtained nanocomposite TiO2/WO3 thin films for photocatalytic applications A. Bojinova *, C. Dushkin, M. Kostadinov, N. Kaneva, G. Ivanova, P. Georgiev
Laboratory of Nanoparticle Science and Technology, Department of General and Inorganic Chemistry, Faculty of Chemistry, University of Sofia, 1 James Bourchier Blvd., 1164 Sofia, Bulgaria * E-mail address:
[email protected] (A. Bojinova).
Abstract. Titania/tungsten oxide nanocomposite thin films are prepared on glass substrates via sol-gel method and dip-coating. The precursor sols contain the respective metal chloride salts, dissolved in ethanol. Hydrogen peroxide is added to the initial reaction mixtures as catalyst for hydrolysis and sol stabilization. The selected WO3 content in the composite films is 5 and 10%. The films are prepared by repetition of 4 coating cycles. The films are dried in air between the successive coating cycles and finally annealed in air at 500°C for 1.5 h. The morphology and phase composition of the composite films is characterized by SEM, AFM and X-ray analysis. The photocatalytic action of the films is checked in photodegradation of the commercial dyes malachite green oxalate and orange II in water solutions under UV and visible light irradiation. The influence of light power on the photocatalityc performance is also investigated. The composite TiO2/WO3 films of 10% WO3 content manifest always higher photocatalytic efficiency than the pure titania films irrespective of the type of irradiation. Keywords: TiO2/WO3 films, visible light photocatalysis, sol-gel process, dip coating 1. Introduction The first report of Fujishima and Honda on UV induced water photolysis at TiO2 electrode [1] provokes remarkable scientific interest in TiO2-based photocatalysis in the next decades. The number of publications have increased dramatically over the past years to the extent of 2400 heterogeneous photochemistry papers published in 2008 [2], among them 80% related to TiO2-based materials. There are numerous reviews on heterogeneous photochemistry [3–5], TiO2 photochemistry [2,6–12], photocatalytic water splitting and hydrogen production [3,13–15], dye sensitization and solar energy conversion [13,16–18] and photochemical air and water treatment [19–21]. It is not surprising that most of the TiO2 photocatalysis reviews are focused on an application-driven perspective, as TiO2 photocatalysis is widely implemented in the environmental cleanup. The most common applications are self-cleaning, anti-fogging and bactericide TiO2-based materials. The future perspective for TiO2 photocatalysis depends mostly on the possibilities for improvement of photocatalysts efficiency under visible light illumination [2–4], especially in case of water purification, where the pollutant concentrations are higher and the light absorption is lower than in air. Doping of TiO2 with different metals or nonmetals is modification approach, used to extend the absorption range of TiO2 to the visible region. The dopant ions introduce energy levels in the TiO2 band gap. Depending on the position of dopants energy levels, they can overlap with the band states, thus narrowing the band gap of the host material. The dopant states can also serve as recombination centers, when their energy lays deep within the host band gap, or in case of high concentrations of the dopant. Improved efficiency is achieved, when the photogenerated carriers can migrate free from the impurity states to the catalysts surface. Such effect has been reported by different research groups, investigating
bicomponent powder catalyst of TiO2/WO3 [22-24]. The TiO2/WO3 material seems promising for visible light induced photocatalysis, due to the suitable combination of the energy band gaps (Eg=3.2 eV for anatase, Eg=2.8 eV for WO3) and the stability of both oxides. The aim of the present work is to obtain photocatalytic films of TiO2/WO3, effective under either UV or visible light irradiation. The films are prepared by sol-gel method and dip-coating and consist of 4 coatings. Here we use for the fist time metal chloride salts, as sources of the respective metal ions, for the preparation of the initial precursor sols. In difference of .our previous study [25], here stable sols are prepared without any addition of organics. The photocatalytic action of the films is checked and compared in degradation of the organic dyes malachite green (MG) and orange II (O II) from water solutions under light irradiation of different power. 2. Experimental The reagents and materials, used in the experiments, were as follows: titanium tetrachloride (TiCl4, Merck); tungsten hexachloride (WCl6, Aldrich); hydrogen peroxide (H2O2, Merck); ethanol (96%, density 0.78 g.cm-3, Institute of Pure Substances-Sofia University); microscopic glass slides (75x26x1 mm, Isolab-Germany). As model pollutants were used the commercial organic dyes malachite green oxalate (C48H50N4O4.2C2H2O4, dye content >90%, λ max=618 nm) or orange II (C16H11N2NaO4S, dye content ~85%, λ max=484 nm), supplied from Aldrich. For preparation of the precursor sols, the metal salts, TiCl4 and WCl6, were dissolved separately in desired amounts of ethanol at constant stirring of 300 rpm. Then 1-2 droplets of H2O2 were added to the Ti4+ and W6+ containing sols. The starting sols preparation was completed in minutes under visual control, due to the significant color changes of the reaction mixtures. After the H2O2 addition, the titanium sol immediately turned orange-red. The coloration is typical for formation of peroxotitanates as Ti(O2)(OH)n-24-n+, where titanium (IV) ions are in coordination with bidentate [O2]2- ligands. During the preparation of tungsten sol at first took place reaction of solvolysis and the solution became yellow. Since ethanol was taken in excess, the replacement of Cl- ions by ethoxide groups was complete in minutes and the color changed from yellow to transparent. Each mol W(OC2H5)6 formed, released 6 mol HCl. The as-generated local acidifications in the tungstate (VI) solution in seconds produced white precipitate of hydrated tungsten (VI) oxide, decomposing partially the tungsten ethoxide. After a few seconds of stirring, the solution turned from light to dark blue, and the tungsten (VI) ethoxide was completely decomposed and formed amorphous WO3.nH2O (n=1÷2) whit a dark blue color as the final colloidal tungsten(VI) sol. The titanium sol was used per se for preparation of bare TiO2 films and also mixed together with the tungsten sol in amounts, corresponding to 5 or 10 wt % WO3 content in the TiO2/WO3 composite films. The obtained mixed sols were transparent and viscous, due to transformation of the blue amorphous WO3.nH2O to soluble peroxotungstic species such as [W2O3(O2)4(H2O)2]2-. The solutions, covered with a watch-glass, were left to age overnight before use. The sols were stable and remain clear - no precipitation occurred within 2 months after preparation. The films were deposited on glass substrates by dip-coating. The withdrawal speed of dipping and lifting of the substrate was 0.9 mm.s-1. The detailed procedure and description of the dip-coater are given elsewhere [26]. The films were dried at 100ºC for 5 min between the successive coatings and finally annealed at 520°C 2 h in air for complete organics decomposition. The as-obtained TiO2 and TiO2/WO3 films were tested for photocatalysis by standard procedure in photodegradation of 5 ppm MG or 7 ppm O II in water solutions under UV or visible-light illumination. The volume of dye solution was 150 ml. The sources of radiation were as follows: UVA lamp (Sylvania 18W BLB T8, emitting mainly in the range 315-400 nm) placed 10 cm above; energy saver lamp OSRAM LUMULUX 21W=100W 1230 Lm, maximal emission 550-600 nm) and a linear Tungsram lamp 500 W K1R7s 9700 Lm, maximal emission at 700 nm) for the visible irradiation fixed at 25 cm above the treated solution. The visible lamps spectra were measured by spectrophotometer
AVANTES. Aliquot samples were regularly taken from the malachite green solution at determined time intervals and analyzed for absorption at the maximal absorption of the dye by spectrophotometer (Jenway 6400). After measurement, the sample was returned back to the treated solution. The solution was constantly stirred by electromagnetic stirrer at a constant rotation speed of 400 rpm. The film thickness was determined by weight method, the morphology – by scanning electron microscope (SEM) JEOL JSM-5510 and atomic force microscope (AFM) MultiMode Veeco, the crystalline phase and parameters - by diffractometer Siemens D 500 (CuKα source-radiation at a step of 0.05 deg for 2Ө and counting time 2s/step). 3. Results and discussion The phase composition and crystallinity of the composite films with 10% WO3 is determined by X-ray diffraction and shown in Figure 1. The film consists of TiO2 in form of anatase (main peak at 25.3° with particle crystallite sizes of 66 nm) and monoclinic WO3 with a characteristic triple peak at 23.15–24.35°. The average size of WO3 crystallites is 61 nm. Both oxides exist in the composite film as separated phases, as seen from the peaks in Figure 1, there is no indication for mixed compound formed. The average film weight for TiO2 and TiO2/WO3 films with 4 coatings, determined after deposition by weight method, is found to be 2.7 and 3.9 mg.cm-2, respectively. Film thickness of 1.5-2 μm for both films is observed by SEM. The morphology of the prepared films is investigated by SEM (Figure 2) and AFM (Figure 3) as well. Tungsten oxide particles are well recognizable at the micrographs in Figure 2c, especially at higher magnifications (Figure 2d). From the SEM images in Figure 2a and 2c is clearly seen that the surface of the films is constituted of divided by cracks island groups. This type of morphology becomes more pronounced with the rise of WO3 content in the films. The cracks in the TiO2/WO3 composite films are wider in comparison to these in the TiO2 films. This observation is in agreement with the determined by AFM surface roughness of the samples, shown in Figure 3. The average roughness is found to be 0.0993 for TiO2 film, 0.125 for film with 5%WO3 and 0.1841 for composite film with 10% WO3 content. The photocatalytic action of TiO2 and TiO2/WO3 composite films is checked in degradation of MG in water solutions irradiated with UV or visible light. The results from the photocatalytic tests after 3 hours photocatalysis are shown in Figure 4. As seen from Figure 4a, the photocatalytic efficiency under UV light is generally increasing with the WO3 content in the films – the degree of dye degradation is 33% for TiO2, 65% for composite with 5% WO3 and 78% for composite with 10% WO3 films. The results are in good correlation with the values for films roughness – the composite TiO2/WO3 films exhibit higher rates of photodegradation and higher roughness as well. Similar trend in the photocatalytic activity is observed also under high power (500 W) visible light (second row in Figure 4b). Here the achieved MG degradation is at least equal (for the TiO2/5%WO3 film), or even more higher – 48% for TiO2 films and 99% for films with 10% WO3, in comparison to photocatalysis with the same samples under UV light. The higher rate of degradation with TiO2/WO3, than that with TiO2, in the case of malachite green can be explained by photoinduced adsorption of the anion part of the particular cationic dye onto the composite catalyst particles due to effective charge separation in the TiO2/WO3 system under light excitation. The data for the energy saver lamp (100 W) show that the increment of the WO3 content in the films enhances the degree of photodegradation at a very small value (first row in Figure 4b). Probably in this case the light intensity is insufficient to activate a substantial visible photocatalysis, as under powerful visible light. As seen, the composite TiO2/WO3 films always achieve a higher degradation degree of MG, than that of the pure TiO2 film, irrespective to the applied source of irradiation (UV or visible). The results from the photocatalytic tests with the dye orange II are presented in Figure 5. Here the photocatalytic action of all the investigated films is almost equal. Although the effect of TiO2/WO3 films in MG degradation is not observed in case of O II, they exhibit the same activity as the pure TiO2
samples. Nevertheless, the pure TiO2 and TiO2/WO3 composite films are proved to be efficient photocatalysts for the destruction of O II under UV (where the degree of dye degradation is higher than that in case of MG – Figure 5a) and visible light. 4. Conclusion We have prepared for first time thin solid films of TiO2/WO3 using all sol-gel process and dip coating. The starting sols, containing only the respective metal peroxocomplexes, are stable within 2 months without any addition of organic stabilizer. The WO3 content in the films is varied as 5 and wt 10%. The films exhibit a good homogeneity and nanocrystallinity as shown by SEM, AFM and XRD investigations. The photocatalytic tests with respect to destruction of MG prove that of best performance is always the composite TiO2/WO3 with 10 wt% WO3. This result is confirmed with both UV and visible light illumination using lamps of different power. Such effect is not observed in case of O II, where all films manifest higher efficiency in O II degradation in comparison to the results in case of MG. Acknowledgements: The authors acknowledge the financial support by project DDVU 02-36/10 of the National Science Fund of Bulgaria and project FP7-REGPOT-2011-1 Beyond Everest. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37-38. [2] A. Fujishima, X. Zhang, D.A. Tryk, Surface Science Reports 63 (2008) 515-582. [3] M. Grätzel, Modern Aspects of Electrochemistry 15 (1983) 83-165. [4] P.V. Kamat, Chemical Reviews 93 (1993) 267-300. [5] A.V. Emeline, V.K. Ryabchuk, N. Serpone, Catalysis Today 122 (2007) 91. [6] X. Chen, S.S. Mao, Chemical Reviews 107 (2007) 2891. [7] J.M. Herrmann, Catalysis Today 53 (1999) 115-129. [8] A. Mills, S. Lehunte, Journal of Photochemistry and Photobiology A 108 (1997) 1-35. [9] M. Kitano, M. Matsuoka, M. Ueshima, M. Anpo, Applied Catalysis A 325 (2007) 1-14. [10] O. Carp, C.L. Huisman, A. Reller, Progress in Solid State Chemistry 32 (2004) 33-177. [11] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chemical Reviews 95 (1995) 69-96. [12] P. Calza, E. Pelizzetti, C. Minero, Journal of Applied Electrochemistry 35 (2005) 665-673. [13] N. Serpone, E. Pelizzetti, M. Grätzel, Coordination Chemistry Reviews 64 (1985) 225-245. [14] A.J. Bard, M.A. Fox, Accounts of Chemical Research 28 (1995) 141-145. [15] M. Kitano, K. Tsujimaru, M. Anpo, Topics in Catalysis 49 (2008) 4-17. [16] P.V. Kamat, Journal of Physical Chemistry C 111 (2007) 2834-2860. [17] M. Grätzel, Inorganic Chemistry 44 (2005) 6841-6851. [18] A. Hagfeldt, G. Boschloo, L.C. Sun, L. Kloo, H. Pettersson, Chemical Reviews 110 (2010) 6595-6663. [19] I.K. Konstantinou, T.A. Albanis, Applied Catalysis B 49 (2004) 1-14. [20] T.L. Thompson, J.T. Yates Jr., Chemical Reviews 106 (2006) 4428-4453. [21] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chemical Reviews 95 (1995) 735-758. [22] V. Puddu, R. Mokaya, G. Li Puma, Chem. Commun. 45 (2007) 4749-4751. [23] O. Lorret, D. Francova, G. Waldner, N. Stelzer, Appl Cat B: Environ 91 (2009) 39–46. [24] A. S. Bojinova, C. I. Papazova, I. B. Karadjova, I. Poulios, Eurasian J. Analyt. Chem. 2 (1) (2008) 34-43. [25] A. Bojinova, C. Dushkin, Nanoscience & Nanotechnology 9, E. Balabanova, I. Dragieva (Eds.) Heron Press Sci. Series, (2009) 84 –87. [26] C. Dushkin, S. Stoyanov, A. Bojinova, S. Russev. Ann. L'Univ. Sof., Fac. Chim. 98-99 (2006) 73-82.
Figure captions Figure 1. XRD pattern of composite film with 10% WO3 content. Figure 2. SEM micrographs of sol-gel prepared films of: (a) and (b) TiO2; (c) and (d) composite film of 10% WO3 content. Figure 3. AFM images of three-dimensional configuration of thin photocatalytic films: (a) TiO2; (b) TiO2/10%WO3; and (c) TiO2/5%WO3 film. Figure 4. Comparison of degree of photocatalytic decomposition of 5-ppm MG in water solution by pure TiO2 and TiO2/WO3 films under: (a) UV illumination: (b) visible light of different power. The experimental data correspond to 3 h of irradiation. Figure 5. Comparison of the photocatalytic action of pure TiO2 and modified TiO2/WO3 films in the decomposition of 7 ppm O II after 3 h of illumination under: (a) UV light; (b) visible light.
Intensity, c/s
Figure 1.
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Figure 2.
Figure 3
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Figure 5.
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