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Photocatalytically active TiO2 thin films produced by surfactant-assisted sol–gel processing
Thin Solid Films 495 (2006) 327 – 332 www.elsevier.com/locate/tsf
Photocatalytically active TiO2 thin films produced by surfactant-assisted sol–gel processing Urh Cˇernigoj a, Urxka Lavren*i* Sˇtangar a,b,*, Polonca Trebxe a, Urxa Opara Kraxovec c, Silvia Gross d a
Laboratory for Environmental Research, Nova Gorica Polytechnic, P.O.B.301, 5001 -Nova Gorica, Slovenia b National Institute of Chemistry, Hajdrihova 19, 1000 -Ljubljana, Slovenia c University of Ljubljana, Faculty of Electrical Engineering, Traxka 25, 1000 -Ljubljana, Slovenia d CNR -ISTM, University of Padova, via Marzolo, I -35131 Padova, Italy Available online 26 September 2005
Abstract Thin TiO2 films were prepared from a titanium isopropoxide precursor by sol – gel processing with or without various nonionic surfactant molecules (Brij 56, Triton X-100 or Pluronic F-127). The photocatalytic efficiency of the transparent films obtained by a dip-coating technique was found to depend strongly on the use of and type of surfactant added. Titania/Pluronic sols resulted in homogeneous and crack-free TiO2 anatase films with a thickness as much as 300 nm after one dipping and heat-treatment (500 -C) cycle. Optical properties of the films were characterized by UV-Vis spectroscopy and crystalline structures by X-ray diffraction. A surfactant-assisted sol – gel process retarded crystallization of the anatase titania films, which resulted in smaller grain sizes (down to 10 nm) and presumably a larger active surface area. The morphology of the film surfaces as obtained by SEM techniques could be also correlated with the results of our photodegradation studies. The photocatalytic activity of the films was enhanced by first coating the glass substrate with a SiO2 protective layer prior to the deposition of the titania film. For our in situ studies of photodegradation we chose the monoazo dye Plasmocorinth B as a model compound as it is stable under environmental conditions and its degradation products are not coloured. The highest photobleaching rate was found for films deposited from the sol with addition of the Pluronic surfactant and it was almost twice as high as that for films deposited from sols without the surfactant. D 2005 Elsevier B.V. All rights reserved. Keywords: Titania coatings; Sol – gel; Photocatalysis; Azo dye
1. Introduction Titania in the form of thin films has attracted a great deal of interest as a low cost and very versatile material for applications in photocatalysis, dye-sensitized photoelectrochemical cells, electrochromic devices, sensors, and many other uses. For many applications a nanostructured titania film is preferable to one which is dense, or compact. Numerous reports have appeared on highly porous titania film structures either with large pores [1] or ordered mesopores [2– 5]. The nanocrystalline structure of the films is usually produced and controlled by surfactant-assisted sol – gel processing [e.g. 6]. Sol –gel techniques offer some advantages compared to other solution methods or gas phase deposition techniques, such as * Corresponding author. Nova Gorica Polytechnic, Vipavska 13, 5000 Nova Gorica, Slovenia. Tel.: +386 5 3315 241; fax: +386 5 3315 296. E-mail address: [email protected] (U.L. Sˇtangar). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.240
ease and versatility of processing, homogeneity at a molecular level, as well as mild and ambient atmosphere reaction conditions. As smaller grain sizes are usually associated with higher surface areas [7– 9], the conventional alkoxide sol –gel route has been modified with the addition of a templating agent to produce such smaller grains. A large surface area can be the determining factor in photodegradation reactions because a large contact surface exposed to organic pollutant molecules promotes the reaction rate [7,9]. However, smaller particles can also promote the recombination of photogenerated electrons and holes which in turn leads to a poor photoactivity. Hence, a balance between surface area and crystallinity must be found in order to obtain the optimum photocatalytic activity [7,10]. A titania particle size of around 10 nm has been suggested to produce the highest photocatalytic oxidation rates [11]. In this work we utilized several commercially available, non-ionic surfactants (Brij 56, Pluronic F-127, Triton X-100) with polyether chains of different lengths by applying them in
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the sol – gel process with the titanium isopropoxide precursor to induce nanocrystallinity in the resulting inorganic films in an attempt to increase their photocatalytic activity for degradation of organic pollutants in waste water. In situ photodegradation of the azo dye Plasmocorinth B in aqueous solution in the presence of various titania thin films was the model system in our study. The reasons for the choice of this particular dye are:(i) it is stable under environmental conditions, (ii) its degradation products are not colored and therefore easy to follow with UV-Vis spectroscopy, (iii) it does not adsorb extensively on the titania film so that the discoloration of the dye solution could be directly related to its decomposition. The photocatalytic efficiency of titania films is also affected by additional factors such as the presence of cations (Na+, Ca2+)which may diffuse from the glass substrate in the interior of the film [12], and have been found to have a detrimental effect on the photocatalytic activity [13,14]. To reduce the migration of cationic impurities into the active film we covered the soda – lime glass substrates with a thin silica film made by a sol – gel route prior to depositing the titania film. Crystallinity of the thin films and powders calcined at 500 -C was examined by XRD analysis. It is known that the anatase crystal structure shows greater photocatalytic activity than rutile form for most reactions. The reason for this is rather unclear. It has been ascribed to a slightly higher Fermi level for anatase, a lower capacity to adsorb oxygen and a higher degree of surface hydroxylation [7]. Hydroxyl groups on the surface have an important role in the photodegradation process since they directly trap photogenerated holes to produce the very reactive OH&radicals which subsequently oxidize the organic pollutant. They also have an indirect role in producing oxygen radicals and hindering the electron-hole recombination. 2. Experimental 2.1. Materials preparation Titania sols were made from a modified Ti(OPri)4 precursor (Fluka) by addition of acetic acid (CH3COOH/Ti = 1.0)(sols U, R, T) or ethyl acetoacetate (EAA/Ti = 1.0)(sols D, C) in order to slow down the solvolysis – condensation of the titanium alkoxide. The resulting exothermic reaction gave rise to a stable solution into which the solvent 2-methoxyethanol was added [15] ((Ti/MeO(CH2)2OH = 0.075)without surfactant (sol U, D)or with surfactant, namely polyoxyethylene(10)cetyl ether (Brij 56, Aldrich)(15 wt.%, sol R), polyoxyethylene(10)isooctylphenyl ether (Triton X-100, Aldrich)(15 wt.%, sol T) or PEO100PPO65PEO100 triblock copolymer (Pluronic F-127, Sigma)(9.7 wt.%, sol C). Surfactant weight percentages are given with respect to the total weight of solution. The resulting solution was stirred at room temperature for at least three hours. The yellowish transparent sols were stable in a refrigerator for several months. Coatings were made in the dip-coating unit with pulling speeds of up to 10 cm/min. The supporting soda – lime glass plates (bare or coated with SiO2)had been cleaned with ethanol and dried. The xerogel films were calcined at 500 -C for 30 to 90 min. The thickness
of the TiO2 films was increased by repeating the dipping and heat-treatment cycle up to five times. Powder samples were prepared by casting the dip-coating solution into petri dishes. After solvent evaporation the remaining thick film was calcined at 500 -C for 2 h. Silica sol was prepared from a tetraethoxysilane (TEOS, Aldrich)precursor, ethanol (Si/EtOH = 0.15), HNO3 (HNO3/ Si = 0.265)and water (H2O/Si = 8.1). After two hours of mixing, the resulting solution was used for silica film deposition by the dip-coating technique with pulling speeds up to 5 cm/min. The sol was unstable and therefore was always freshly prepared. All silica films were produced by a single dipping. The silica films on soda – lime glass were used as substrates for the deposition of titania thin films as described above. 2.2. Characterization techniques The thickness of the films was measured on a TaylorHobson Talysurf profilometer. UV-Vis spectra of the films and the dye solutions were recorded on a Hewlett-Packard 8453 UV-Vis spectrophotometer. X-ray diffraction measurements were made with a Philips PW1710 automated X-ray diffractometer using graphite monochromatized Cu Ka radiation in the step-by-step mode. SEM images of the films were obtained on a Supra 35 VP Carl Zeiss field-emission scanning electron microscope with an accelerating voltage of 1 kV. The surface and in-depth composition of films were analysed by X-ray Photoelectron Spectroscopy (XPS). XPS spectra were run on a Perkin-Elmer A 5600 ci spectrometer. A continuous flow reactor for the experiments on the photocatalytic activity of the films was constructed. The reactor could be purged with different gases, was cooled with tap water and had on-line measuring capability of the UV-Vis spectra of the solution. The as-prepared titania films were kept in deionised water prior to the photocatalysis experiments. A 10 mm thick solution of NaBr (110 g) and Pb (NO3)2 (0.69 g) in water (230 g) was used as a 335 nm cut-off filter in front of the photocatalytic cell and a 125 W Xe lamp (Cermax xenon parabolic lamp) as a light source. A titania film on one side of a silica covered glass support (25 70 mm) was immersed in the dye solution next to the wall of the photocatalytic cell and irradiated (surface 23 23 mm) along the normal direction. The dye solution (aqueous solution (4.8 mL) of Plasmocorinth B (40 mg/L)) was continuously purged with oxygen during the irradiation. A peristaltic pump (Heidolph PD 5001) with a silicon hose was used to drive the solution from the photocatalysis cell to the cell positioned in the UV-Vis spectrophotometer for on-line absorbance measurements and back to the photocatalytic cell at a flow rate of 10 mL/min. 3. Results and discussion 3.1. Thickness and optical quality of the films All titania films obtained after calcination were of a high optical quality (transparent, crack-free) and free of any organic residues as confirmed by IR spectroscopic measure-
U. Cˇernigoj et al. / Thin Solid Films 495 (2006) 327 – 332 1600
Film thickness (nm)
C
C on SiO2-glass R on SiO2-glass R on bare glass U on bare glass U on SiO2-glass D on SiO2-glass
1400 1200 1000 800
R
600 400
U,D
200 0 1
2
3
4
5
Number of dippings Fig. 1. Relationship between film thickness and number of dippings from various sols.
ments of the films deposited on Si-wafers. Fig. 1 shows the dip-coating efficiency of the titania films made from different sols. It is apparent that the addition of surfactant in the sol results in a thicker film obtained by the same number of dipping and heat-treatment cycles. This is a consequence of increased viscosity of the sol upon addition of a polymeric templating agent. The highest efficiency was achieved by using Pluronic (Brij and Triton gave similar results), which has the longest polymeric chains. Film thickness obtained by single dipping could be as high as 300 nm without deterioration of the optical quality. A linear increase in film thickness with the number of dippings was observed in all cases. It is also evident from Fig. 1 that type of the substrate (bare glass or SiO2-coated glass) has no influence on the resulting thickness of the titania film. UV-Vis transmittance spectra of various films prepared with four dippings are presented on Fig. 2. The transparency of all films is high and the film with greatest thickness (ca.1200 nm on both sides, curve C) still has a transmittance of around 80%
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over the whole visible light region. Bands from the interference color of the films appeared in the wavelength range 350– 800 nm and have higher amplitudes in the case of thinner films made without surfactant. Similar but much stronger effect was observed for TiO2/PDMS hybrid films, where the interference peaks were even eliminated by the addition of PDMS, which increased surface roughness and porosity of titania [16]. Accordingly, such phenomena in UV-VIS spectra can be ascribed to the change in density and hence in porosity of the film structure. When our samples are compared, the films made by using templates possess higher porosity (which is also discussed in the next section) and therefore their UV-VIS spectra are characterized by less pronounced interference peaks (Fig. 2, C and R spectra). In addition it can be observed that interference fringes differ even inside one particular spectrum (e.g. spectrum C) suggesting that films are multilayers with various refractive indexes.This may be due to repeated coating –heating cycles, where the first layer underwent longer heat-treatment time than the second one and so on. It should be also noted that titania films are deposited on glass with a thin SiO2 protective layer, which may also contribute to the observed phenomenon. Inset in Fig. 2 shows the UV absorption spectra of films deposited from sol C which contained Pluronic surfactant. It is clearly seen that an increase in the number of dippings produced a red-shift of the absorption edge of the film, which was also observed for all other films in agreement with other preparation methods [12,17,18]. The shift has been ascribed to differences in the size of the crystallites. Thicker films that underwent longer heat treatment by repeated coating – heating cycles have relatively larger anatase crystallites and this causes the onset of absorption to shift to the red part of the spectrum [4]. Absorption of light below 370 nm is due to the excitation of electrons from the valence band to the conduction band of TiO2. Pure anatase crystalline powder has an intense absorption band at 335 nm [19]. A red-shift of the absorption edge also
Fig. 2. UV-Vis transmittance spectra of TiO2 films deposited from various sols on SiO2-glass substrates with 4 dipping – heating cycles. Inset: UV-Vis absorbance spectra of TiO2 films deposited from sol C with 1 to 5 dipping – heating cycles.
U. Cˇernigoj et al. / Thin Solid Films 495 (2006) 327 – 332
330
(101)
180 160
120
(004)
Intensity (a.u.)
140
C
100
D
80
S
60 40 20
30
40
60
50
2θ (degree) Fig. 3. XRD patterns of TiO2 films deposited from sol C (with Pluronic) and D (without Pluronic) on SiO2-glass substrate with 4 dipping – heating cycles (S: XRD pattern of the substrate).
indicates a decrease in the band gap of TiO2 with increasing number of coating –heating cycles. 3.2. Crystalline structure and morphology Typical XRD diffraction patterns of films deposited on SiO2-coated glass substrates are shown in Fig. 3. The only one major peak at 2u = 25.4- corresponds to (101) reflections of the anatase phase, which is present in all our thin film samples. The broad background originates from the glass substrate. The diffraction pattern of a film produced without surfactant added to the titania sol revealed a peak at 2u = 37.9-, which corresponds to (004) reflections of anatase. These observations show that both the orientation and the growth of anatase crystallites in the solid film are influenced by the addition of surfactant in the initial sol. The use of surfactant contributes to the formation of highly oriented anatase phase along the (101) crystallographic plane and to the smaller mean grain size for films and powders (Table 1). The grain size, i.e. the effective size of coherently scattering domains, was calculated from the (101) peak of anatase using the Scherrer’s equation: L = K k / bcos u [20]. Here k is the wavelength of X-rays (1.54 A for Cu K a radiation), K is a constant taken to be 0.9 (assuming no crystal distortion in the lattice), b is the full width at half maximum of the signal in radians corrected for the instrumental broadening (0.07-), and u is the diffraction angle in degrees. A crystallite size of 10 –11 nm on SiO2-glass and 13 –15 nm on
bare glass support was thereby estimated for films prepared with surfactant. The slightly larger anatase grains of films deposited on bare glass substrates may be a consequence of the higher concentration of Na+ ions diffused from the substrate. Indeed, the first results obtained by X-ray photoelectron spectroscopy showed that a relevant atomic percentage of sodium (4 –5%) was detected along the film thickness of titania deposited on bare glass support, while the concentration of alkali metal ions in titania film deposited on SiO2-glass was under detection limit. Na+ ions have been found to act as a flux material for crystal growth [21]. It has been also suggested that diffusion of sodium ions from the substrate during thermal treatment could stimulate the recrystallization of the anatase to rutile [22]. Our studies suggest that crystallization is slower for the surfactant-assisted process compared with that in surfactant-free systems. Smaller grains favour a larger inner surface area, which we achieved effectively with the use of surfactant. BET surface area measurements carried out on powder samples gave preliminary values of 122 m2/g for TiO2 prepared in the presence of Triton and 18 m2/g when prepared without surfactant. Larger crystallites were found for powder samples (Table 1), where the estimated grain size was 17– 23 nm for surfactantassisted process and 29 nm for surfactant-free process of powder preparation. The diffraction patterns of the calcined powder samples also showed the presence of the rutile phase in agreement with the results obtained by Martyanov and Klabunde [18]. These authors made a comparative study of TiO2 particles in powder form and as a thin nanostructured film on quartz. They pointed out that TiO2 particles immobilized on the quartz support remained in the anatase form even at 800 -C, whereas self -supported TiO2 particles in powders were more easily converted into the rutile form starting already at 500 -C. The percentage content of rutile phase relative to anatase (calculated from the intensity ratio I rutile(110) / I anatase(101)) in our powder samples decreased from the highest value of 31%for titania made without surfactant to 16% (with Triton), 9% (with Brij)and to the lowest value of 5% for titania made with Pluronic surfactant.Pluronic was therefore found to have the biggest impact by the greatest retardation of the formation of the rutile phase and by producing the smallest size of anatase
Table 1 The effective size of coherently scattering domains (=grain size)of TiO2 powder (p) and film (f) samples (calculated using Scherrer’s equation) Calcination time, sample form
2h, p. 4 30V, f. on SiO2-glass 4 30V, f. on glass
Average grain size (nm) for samples from different sols: U (no surfactant)
T (Triton)
R (Brij)
C (Pluronic)
29 18 17
22
23 11 15
17 10 13
Fig. 4. SEM image of TiO2 film deposited from sol C on SiO2-glass substrate with 4 dipping – heating cycles.
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nanocrystals. However, no ordered mesoporous structure was obtained as would have been evident by the appearance of a low -angle XRD peak. Scanning electron microscopy (SEM)was employed to characterize the surface morphology of the manufactured films. TiO2 films made without surfactant addition in the sol exhibit a flat and ill-defined surface, whereas TiO2 films made with surfactant addition in the sol (Fig. 4 with Pluronic) exhibit a rougher but still moderately flat texture and well-structured granular nanosurface with clear interstices between the particles/aggregates. Crystallite sizes obtained from XRD measurements (10 nm) are only roughly comparable to the particle size obtained from SEM photographs. The wellstructured morphology displayed in the SEM image of templated films is likely a consequence of template removal during the thermal treatment of thin films. It creates a higher degree of porosity, which should have a beneficial influence on the photocatalytic activity of the templated films provided that the pores are accessible for an organic pollutant. The difference in porosity among the various films studied was indirectly estimated from the densities of the films obtained by taking into account the film thickness and its weight. It was found that Pluronic-templated film (2200 g/dm3)was only half as dense as the film prepared in the absence of surfactant (4300 g/dm3). The former one could be therefore considered as much more porous than the latter one. All coatings were crack-free and uniform on a micrometer scale. A rougher and more porous surface can play an important role in the photocatalytic activity since the photocatalysis reaction occurs at the surface, and a larger surface area provides for more photocatalysis reactions to take place. 3.3. Photodegradation An aqueous solution of Plasmocorinth azo dye (chemical structure is shown in Fig. 5 as an inset) was used for photodegradation studies in the continuous flow cell described above. The dye solution was stable in the absence of the TiO2
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photocatalyst under the irradiation conditions used for all photocatalysis experiments. Fig. 5 shows photobleaching curves (absorbance at 527 nm as a function of irradiation time) in aqueous solution of Plasmocorinth B in the presence of various titania films made by four coating – heating cycles. Only one side of the TiO2/ SiO2-glass slide was in contact with the solution. It is clearly seen that the film made with Pluronic surfactant added showed the highest photodegradation rates compared with the other films with the same number of layers. The dye solution with Pluronic –titania based films was almost completely discolored in less than 1 h of irradiation. All photobleaching curves indicate first-order kinetics. The highest photodegradation rate constant (7.3 10 4 s 1 for films made from sol C with 4 dippings) was almost twice that for films deposited from sols without the surfactant (3.8 10 4 s 1 for films made from sol D with 4 dippings). The reasons for the Pluronic – titania based films having the highest efficiency are that (i) the films are thicker than the others, which results in an increase of the UV absorbance, and that (ii) the grains are smaller and exhibit a well-structured surface, which gives in higher photochemical reaction rates because of the increased active surface area. An appropriate balance between the two factors should be achieved to obtain highest photoefficiency. First, while films have to be sufficiently thick to achieve sufficient catalyst loading, opacity and internal mass transfer resistance is also increased with increased thickness. This could lead to higher electron/hole recombination rates. Aggregation and growth of TiO2 particles in the interior region of thick films, which underwent longer calcinations times, may also cause a decrease in the number of surface active sites [12]. Second, as far as the optimal grain size is concerned, smaller grains result in a higher active surface area, but electron-hole recombination rates are also higher for the smaller particles. 4. Summary Nanostructured and transparent titania films with anatase grain sizes of as small as 10 nm were deposited on SiO2 precoated soda – lime glass substrates in order to enhance their activity for the photocatalytic degradation of an azo-dye (Plasmocorinth B). The use of the Pluronic F-127 templating agent in a surfactant-assisted sol –gel process reduced the number of dip coatings necessary to obtain a sufficient titania loading with photocatalytically active surface area. The films were immersed in the colored solution and photobleaching was followed in situ to near completion in one hour. The titania catalyst can be easily removed from the solution, which is one of the principal advantages of using the immobilized films as catalysts rather than titania powders. Acknowledgements
Fig. 5. Photobleaching of Plasmocorinth B solution (absorbance at 527 nm vs. irradiation time) in the absence and presence of TiO2 films deposited from various sols on SiO2-glass substrates with 4 dipping – heating cycles.
Authors wish to thank Prof. B. Orel and Prof. V. Kau*i* for valuable discussions and XRD measurements, respectively, Dr. M. Cˇekada for thickness measurements, D. Strm*nik and M. Zorko for SEM and Dr. K. Dahmouche for BET surface
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area measurements. We are very grateful to Prof. J. Eckert for revising the paper. This work was financially supported by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia. References [1] S. Bu, Z. Jin, X. Liu, H. Du, Z. Cheng, J. Sol – Gel Sci. Technol. 30 (2004) 239. [2] E.L. Crepaldi, G.J. de A.A Soler-Illia, D.D. Grosso, F. Cagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770. [3] P.C.A. Alberius, K.L. Frindell, R.C. Hayward, E.J. Kramer, G.D Stucky, B.F. Chmelka, Chem. Mater. 14 (2002) 3284. [4] J.C. Yu, X. Wang, X. Fu, Chem. Mater. 16 (2004) 1523. [5] H. Yun, K. Miyazawa, I. Honma, H. Zhou, M. Kuwabara, Mater. Sci. Eng., C 23 (2003) 487. [6] E. Stathatos, P. Lianos, C. Tsakiroglou, Langmuir 20 (2004) 9103. [7] O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32 (2004) 33. [8] P. Bouras, E. Stathatos, P. Lianos, C. Tsakiroglou, Appl. Catal., B Environ. 51 (2004) 275. [9] K. Mogyoro´si, J. Ne´meth, I. De´ka´ny, J.H. Fendler, Prog. Colloid & Polym. Sci. 117 (2001) 88.
[10] K. Mogyoro´si, I. De´ka´ny, J.H. Fendler, Langmuir 19 (2003) 2938. [11] Z.B. Zhang, C.C. Wang, R. Zakaria, J.Y. Ying, J. Phys. Chem., B 102 (1998) 10871. [12] J. Yu, X. Zhao, Q. Zhao, Mater. Chem. Phys. 69 (2001) 25. [13] C. Guillard, B. Beaugiraud, C. Dutriez, J.-M. Herrmann, H. Jaffrezic, N.J. -Renault, M. Lacroix, Appl. Catal., B Environ. 39 (2002) 331. [14] Y. Yang, X. Li, J. Chen, L. Wang, J. Photochem. Photobiol., A Chem. 163 (2004) 517. [15] U.L. Sˇtangar, B. Orel, A. Re´gis, Ph. Colomban, J. Sol – Gel Sci. Technol. 8 (1997) 965. [16] K. Iketani, R.-D. Sun, M. Toki, K. Hirota, O. Yamaguchi, J. Phys. Chem. Solids 64 (2003) 507. [17] C. Guillard, D. Debayle, A. Gagnaire, H. Jaffrezic, J.-M. Herrmann, Mater. Res. Bull. 39 (2004) 1445. [18] I.N. Martyanov, K.J. Klabunde, J. Catal. 225 (2004) 408. [19] H. Yoshitake, T. Sugihara, T. Tatsumi, Chem. Mater. 14 (2002) 1023. [20] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, 2nd edR, John Wiley and Sons, 1973, p. 687. [21] H.-J. Nam, T. Amemiya, M. Murabayashi, K. Itoh, J. Phys. Chem., B 108 (2004) 8254. [22] C.C. Trapalis, P. Keivanidis, G. Kordas, M. Zaharescu, M. Crisan, A. Szatvanyi, M. Gartner, Thin Solid Films 433 (2003) 186.
Photocatalytically active TiO2 thin films produced by surfactant-assisted sol–gel processing Urh Cˇernigoj a, Urxka Lavren*i* Sˇtangar a,b,*, Polonca Trebxe a, Urxa Opara Kraxovec c, Silvia Gross d a
Laboratory for Environmental Research, Nova Gorica Polytechnic, P.O.B.301, 5001 -Nova Gorica, Slovenia b National Institute of Chemistry, Hajdrihova 19, 1000 -Ljubljana, Slovenia c University of Ljubljana, Faculty of Electrical Engineering, Traxka 25, 1000 -Ljubljana, Slovenia d CNR -ISTM, University of Padova, via Marzolo, I -35131 Padova, Italy Available online 26 September 2005
Abstract Thin TiO2 films were prepared from a titanium isopropoxide precursor by sol – gel processing with or without various nonionic surfactant molecules (Brij 56, Triton X-100 or Pluronic F-127). The photocatalytic efficiency of the transparent films obtained by a dip-coating technique was found to depend strongly on the use of and type of surfactant added. Titania/Pluronic sols resulted in homogeneous and crack-free TiO2 anatase films with a thickness as much as 300 nm after one dipping and heat-treatment (500 -C) cycle. Optical properties of the films were characterized by UV-Vis spectroscopy and crystalline structures by X-ray diffraction. A surfactant-assisted sol – gel process retarded crystallization of the anatase titania films, which resulted in smaller grain sizes (down to 10 nm) and presumably a larger active surface area. The morphology of the film surfaces as obtained by SEM techniques could be also correlated with the results of our photodegradation studies. The photocatalytic activity of the films was enhanced by first coating the glass substrate with a SiO2 protective layer prior to the deposition of the titania film. For our in situ studies of photodegradation we chose the monoazo dye Plasmocorinth B as a model compound as it is stable under environmental conditions and its degradation products are not coloured. The highest photobleaching rate was found for films deposited from the sol with addition of the Pluronic surfactant and it was almost twice as high as that for films deposited from sols without the surfactant. D 2005 Elsevier B.V. All rights reserved. Keywords: Titania coatings; Sol – gel; Photocatalysis; Azo dye
1. Introduction Titania in the form of thin films has attracted a great deal of interest as a low cost and very versatile material for applications in photocatalysis, dye-sensitized photoelectrochemical cells, electrochromic devices, sensors, and many other uses. For many applications a nanostructured titania film is preferable to one which is dense, or compact. Numerous reports have appeared on highly porous titania film structures either with large pores [1] or ordered mesopores [2– 5]. The nanocrystalline structure of the films is usually produced and controlled by surfactant-assisted sol – gel processing [e.g. 6]. Sol –gel techniques offer some advantages compared to other solution methods or gas phase deposition techniques, such as * Corresponding author. Nova Gorica Polytechnic, Vipavska 13, 5000 Nova Gorica, Slovenia. Tel.: +386 5 3315 241; fax: +386 5 3315 296. E-mail address: [email protected] (U.L. Sˇtangar). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.240
ease and versatility of processing, homogeneity at a molecular level, as well as mild and ambient atmosphere reaction conditions. As smaller grain sizes are usually associated with higher surface areas [7– 9], the conventional alkoxide sol –gel route has been modified with the addition of a templating agent to produce such smaller grains. A large surface area can be the determining factor in photodegradation reactions because a large contact surface exposed to organic pollutant molecules promotes the reaction rate [7,9]. However, smaller particles can also promote the recombination of photogenerated electrons and holes which in turn leads to a poor photoactivity. Hence, a balance between surface area and crystallinity must be found in order to obtain the optimum photocatalytic activity [7,10]. A titania particle size of around 10 nm has been suggested to produce the highest photocatalytic oxidation rates [11]. In this work we utilized several commercially available, non-ionic surfactants (Brij 56, Pluronic F-127, Triton X-100) with polyether chains of different lengths by applying them in
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the sol – gel process with the titanium isopropoxide precursor to induce nanocrystallinity in the resulting inorganic films in an attempt to increase their photocatalytic activity for degradation of organic pollutants in waste water. In situ photodegradation of the azo dye Plasmocorinth B in aqueous solution in the presence of various titania thin films was the model system in our study. The reasons for the choice of this particular dye are:(i) it is stable under environmental conditions, (ii) its degradation products are not colored and therefore easy to follow with UV-Vis spectroscopy, (iii) it does not adsorb extensively on the titania film so that the discoloration of the dye solution could be directly related to its decomposition. The photocatalytic efficiency of titania films is also affected by additional factors such as the presence of cations (Na+, Ca2+)which may diffuse from the glass substrate in the interior of the film [12], and have been found to have a detrimental effect on the photocatalytic activity [13,14]. To reduce the migration of cationic impurities into the active film we covered the soda – lime glass substrates with a thin silica film made by a sol – gel route prior to depositing the titania film. Crystallinity of the thin films and powders calcined at 500 -C was examined by XRD analysis. It is known that the anatase crystal structure shows greater photocatalytic activity than rutile form for most reactions. The reason for this is rather unclear. It has been ascribed to a slightly higher Fermi level for anatase, a lower capacity to adsorb oxygen and a higher degree of surface hydroxylation [7]. Hydroxyl groups on the surface have an important role in the photodegradation process since they directly trap photogenerated holes to produce the very reactive OH&radicals which subsequently oxidize the organic pollutant. They also have an indirect role in producing oxygen radicals and hindering the electron-hole recombination. 2. Experimental 2.1. Materials preparation Titania sols were made from a modified Ti(OPri)4 precursor (Fluka) by addition of acetic acid (CH3COOH/Ti = 1.0)(sols U, R, T) or ethyl acetoacetate (EAA/Ti = 1.0)(sols D, C) in order to slow down the solvolysis – condensation of the titanium alkoxide. The resulting exothermic reaction gave rise to a stable solution into which the solvent 2-methoxyethanol was added [15] ((Ti/MeO(CH2)2OH = 0.075)without surfactant (sol U, D)or with surfactant, namely polyoxyethylene(10)cetyl ether (Brij 56, Aldrich)(15 wt.%, sol R), polyoxyethylene(10)isooctylphenyl ether (Triton X-100, Aldrich)(15 wt.%, sol T) or PEO100PPO65PEO100 triblock copolymer (Pluronic F-127, Sigma)(9.7 wt.%, sol C). Surfactant weight percentages are given with respect to the total weight of solution. The resulting solution was stirred at room temperature for at least three hours. The yellowish transparent sols were stable in a refrigerator for several months. Coatings were made in the dip-coating unit with pulling speeds of up to 10 cm/min. The supporting soda – lime glass plates (bare or coated with SiO2)had been cleaned with ethanol and dried. The xerogel films were calcined at 500 -C for 30 to 90 min. The thickness
of the TiO2 films was increased by repeating the dipping and heat-treatment cycle up to five times. Powder samples were prepared by casting the dip-coating solution into petri dishes. After solvent evaporation the remaining thick film was calcined at 500 -C for 2 h. Silica sol was prepared from a tetraethoxysilane (TEOS, Aldrich)precursor, ethanol (Si/EtOH = 0.15), HNO3 (HNO3/ Si = 0.265)and water (H2O/Si = 8.1). After two hours of mixing, the resulting solution was used for silica film deposition by the dip-coating technique with pulling speeds up to 5 cm/min. The sol was unstable and therefore was always freshly prepared. All silica films were produced by a single dipping. The silica films on soda – lime glass were used as substrates for the deposition of titania thin films as described above. 2.2. Characterization techniques The thickness of the films was measured on a TaylorHobson Talysurf profilometer. UV-Vis spectra of the films and the dye solutions were recorded on a Hewlett-Packard 8453 UV-Vis spectrophotometer. X-ray diffraction measurements were made with a Philips PW1710 automated X-ray diffractometer using graphite monochromatized Cu Ka radiation in the step-by-step mode. SEM images of the films were obtained on a Supra 35 VP Carl Zeiss field-emission scanning electron microscope with an accelerating voltage of 1 kV. The surface and in-depth composition of films were analysed by X-ray Photoelectron Spectroscopy (XPS). XPS spectra were run on a Perkin-Elmer A 5600 ci spectrometer. A continuous flow reactor for the experiments on the photocatalytic activity of the films was constructed. The reactor could be purged with different gases, was cooled with tap water and had on-line measuring capability of the UV-Vis spectra of the solution. The as-prepared titania films were kept in deionised water prior to the photocatalysis experiments. A 10 mm thick solution of NaBr (110 g) and Pb (NO3)2 (0.69 g) in water (230 g) was used as a 335 nm cut-off filter in front of the photocatalytic cell and a 125 W Xe lamp (Cermax xenon parabolic lamp) as a light source. A titania film on one side of a silica covered glass support (25 70 mm) was immersed in the dye solution next to the wall of the photocatalytic cell and irradiated (surface 23 23 mm) along the normal direction. The dye solution (aqueous solution (4.8 mL) of Plasmocorinth B (40 mg/L)) was continuously purged with oxygen during the irradiation. A peristaltic pump (Heidolph PD 5001) with a silicon hose was used to drive the solution from the photocatalysis cell to the cell positioned in the UV-Vis spectrophotometer for on-line absorbance measurements and back to the photocatalytic cell at a flow rate of 10 mL/min. 3. Results and discussion 3.1. Thickness and optical quality of the films All titania films obtained after calcination were of a high optical quality (transparent, crack-free) and free of any organic residues as confirmed by IR spectroscopic measure-
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Film thickness (nm)
C
C on SiO2-glass R on SiO2-glass R on bare glass U on bare glass U on SiO2-glass D on SiO2-glass
1400 1200 1000 800
R
600 400
U,D
200 0 1
2
3
4
5
Number of dippings Fig. 1. Relationship between film thickness and number of dippings from various sols.
ments of the films deposited on Si-wafers. Fig. 1 shows the dip-coating efficiency of the titania films made from different sols. It is apparent that the addition of surfactant in the sol results in a thicker film obtained by the same number of dipping and heat-treatment cycles. This is a consequence of increased viscosity of the sol upon addition of a polymeric templating agent. The highest efficiency was achieved by using Pluronic (Brij and Triton gave similar results), which has the longest polymeric chains. Film thickness obtained by single dipping could be as high as 300 nm without deterioration of the optical quality. A linear increase in film thickness with the number of dippings was observed in all cases. It is also evident from Fig. 1 that type of the substrate (bare glass or SiO2-coated glass) has no influence on the resulting thickness of the titania film. UV-Vis transmittance spectra of various films prepared with four dippings are presented on Fig. 2. The transparency of all films is high and the film with greatest thickness (ca.1200 nm on both sides, curve C) still has a transmittance of around 80%
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over the whole visible light region. Bands from the interference color of the films appeared in the wavelength range 350– 800 nm and have higher amplitudes in the case of thinner films made without surfactant. Similar but much stronger effect was observed for TiO2/PDMS hybrid films, where the interference peaks were even eliminated by the addition of PDMS, which increased surface roughness and porosity of titania [16]. Accordingly, such phenomena in UV-VIS spectra can be ascribed to the change in density and hence in porosity of the film structure. When our samples are compared, the films made by using templates possess higher porosity (which is also discussed in the next section) and therefore their UV-VIS spectra are characterized by less pronounced interference peaks (Fig. 2, C and R spectra). In addition it can be observed that interference fringes differ even inside one particular spectrum (e.g. spectrum C) suggesting that films are multilayers with various refractive indexes.This may be due to repeated coating –heating cycles, where the first layer underwent longer heat-treatment time than the second one and so on. It should be also noted that titania films are deposited on glass with a thin SiO2 protective layer, which may also contribute to the observed phenomenon. Inset in Fig. 2 shows the UV absorption spectra of films deposited from sol C which contained Pluronic surfactant. It is clearly seen that an increase in the number of dippings produced a red-shift of the absorption edge of the film, which was also observed for all other films in agreement with other preparation methods [12,17,18]. The shift has been ascribed to differences in the size of the crystallites. Thicker films that underwent longer heat treatment by repeated coating – heating cycles have relatively larger anatase crystallites and this causes the onset of absorption to shift to the red part of the spectrum [4]. Absorption of light below 370 nm is due to the excitation of electrons from the valence band to the conduction band of TiO2. Pure anatase crystalline powder has an intense absorption band at 335 nm [19]. A red-shift of the absorption edge also
Fig. 2. UV-Vis transmittance spectra of TiO2 films deposited from various sols on SiO2-glass substrates with 4 dipping – heating cycles. Inset: UV-Vis absorbance spectra of TiO2 films deposited from sol C with 1 to 5 dipping – heating cycles.
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330
(101)
180 160
120
(004)
Intensity (a.u.)
140
C
100
D
80
S
60 40 20
30
40
60
50
2θ (degree) Fig. 3. XRD patterns of TiO2 films deposited from sol C (with Pluronic) and D (without Pluronic) on SiO2-glass substrate with 4 dipping – heating cycles (S: XRD pattern of the substrate).
indicates a decrease in the band gap of TiO2 with increasing number of coating –heating cycles. 3.2. Crystalline structure and morphology Typical XRD diffraction patterns of films deposited on SiO2-coated glass substrates are shown in Fig. 3. The only one major peak at 2u = 25.4- corresponds to (101) reflections of the anatase phase, which is present in all our thin film samples. The broad background originates from the glass substrate. The diffraction pattern of a film produced without surfactant added to the titania sol revealed a peak at 2u = 37.9-, which corresponds to (004) reflections of anatase. These observations show that both the orientation and the growth of anatase crystallites in the solid film are influenced by the addition of surfactant in the initial sol. The use of surfactant contributes to the formation of highly oriented anatase phase along the (101) crystallographic plane and to the smaller mean grain size for films and powders (Table 1). The grain size, i.e. the effective size of coherently scattering domains, was calculated from the (101) peak of anatase using the Scherrer’s equation: L = K k / bcos u [20]. Here k is the wavelength of X-rays (1.54 A for Cu K a radiation), K is a constant taken to be 0.9 (assuming no crystal distortion in the lattice), b is the full width at half maximum of the signal in radians corrected for the instrumental broadening (0.07-), and u is the diffraction angle in degrees. A crystallite size of 10 –11 nm on SiO2-glass and 13 –15 nm on
bare glass support was thereby estimated for films prepared with surfactant. The slightly larger anatase grains of films deposited on bare glass substrates may be a consequence of the higher concentration of Na+ ions diffused from the substrate. Indeed, the first results obtained by X-ray photoelectron spectroscopy showed that a relevant atomic percentage of sodium (4 –5%) was detected along the film thickness of titania deposited on bare glass support, while the concentration of alkali metal ions in titania film deposited on SiO2-glass was under detection limit. Na+ ions have been found to act as a flux material for crystal growth [21]. It has been also suggested that diffusion of sodium ions from the substrate during thermal treatment could stimulate the recrystallization of the anatase to rutile [22]. Our studies suggest that crystallization is slower for the surfactant-assisted process compared with that in surfactant-free systems. Smaller grains favour a larger inner surface area, which we achieved effectively with the use of surfactant. BET surface area measurements carried out on powder samples gave preliminary values of 122 m2/g for TiO2 prepared in the presence of Triton and 18 m2/g when prepared without surfactant. Larger crystallites were found for powder samples (Table 1), where the estimated grain size was 17– 23 nm for surfactantassisted process and 29 nm for surfactant-free process of powder preparation. The diffraction patterns of the calcined powder samples also showed the presence of the rutile phase in agreement with the results obtained by Martyanov and Klabunde [18]. These authors made a comparative study of TiO2 particles in powder form and as a thin nanostructured film on quartz. They pointed out that TiO2 particles immobilized on the quartz support remained in the anatase form even at 800 -C, whereas self -supported TiO2 particles in powders were more easily converted into the rutile form starting already at 500 -C. The percentage content of rutile phase relative to anatase (calculated from the intensity ratio I rutile(110) / I anatase(101)) in our powder samples decreased from the highest value of 31%for titania made without surfactant to 16% (with Triton), 9% (with Brij)and to the lowest value of 5% for titania made with Pluronic surfactant.Pluronic was therefore found to have the biggest impact by the greatest retardation of the formation of the rutile phase and by producing the smallest size of anatase
Table 1 The effective size of coherently scattering domains (=grain size)of TiO2 powder (p) and film (f) samples (calculated using Scherrer’s equation) Calcination time, sample form
2h, p. 4 30V, f. on SiO2-glass 4 30V, f. on glass
Average grain size (nm) for samples from different sols: U (no surfactant)
T (Triton)
R (Brij)
C (Pluronic)
29 18 17
22
23 11 15
17 10 13
Fig. 4. SEM image of TiO2 film deposited from sol C on SiO2-glass substrate with 4 dipping – heating cycles.
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nanocrystals. However, no ordered mesoporous structure was obtained as would have been evident by the appearance of a low -angle XRD peak. Scanning electron microscopy (SEM)was employed to characterize the surface morphology of the manufactured films. TiO2 films made without surfactant addition in the sol exhibit a flat and ill-defined surface, whereas TiO2 films made with surfactant addition in the sol (Fig. 4 with Pluronic) exhibit a rougher but still moderately flat texture and well-structured granular nanosurface with clear interstices between the particles/aggregates. Crystallite sizes obtained from XRD measurements (10 nm) are only roughly comparable to the particle size obtained from SEM photographs. The wellstructured morphology displayed in the SEM image of templated films is likely a consequence of template removal during the thermal treatment of thin films. It creates a higher degree of porosity, which should have a beneficial influence on the photocatalytic activity of the templated films provided that the pores are accessible for an organic pollutant. The difference in porosity among the various films studied was indirectly estimated from the densities of the films obtained by taking into account the film thickness and its weight. It was found that Pluronic-templated film (2200 g/dm3)was only half as dense as the film prepared in the absence of surfactant (4300 g/dm3). The former one could be therefore considered as much more porous than the latter one. All coatings were crack-free and uniform on a micrometer scale. A rougher and more porous surface can play an important role in the photocatalytic activity since the photocatalysis reaction occurs at the surface, and a larger surface area provides for more photocatalysis reactions to take place. 3.3. Photodegradation An aqueous solution of Plasmocorinth azo dye (chemical structure is shown in Fig. 5 as an inset) was used for photodegradation studies in the continuous flow cell described above. The dye solution was stable in the absence of the TiO2
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photocatalyst under the irradiation conditions used for all photocatalysis experiments. Fig. 5 shows photobleaching curves (absorbance at 527 nm as a function of irradiation time) in aqueous solution of Plasmocorinth B in the presence of various titania films made by four coating – heating cycles. Only one side of the TiO2/ SiO2-glass slide was in contact with the solution. It is clearly seen that the film made with Pluronic surfactant added showed the highest photodegradation rates compared with the other films with the same number of layers. The dye solution with Pluronic –titania based films was almost completely discolored in less than 1 h of irradiation. All photobleaching curves indicate first-order kinetics. The highest photodegradation rate constant (7.3 10 4 s 1 for films made from sol C with 4 dippings) was almost twice that for films deposited from sols without the surfactant (3.8 10 4 s 1 for films made from sol D with 4 dippings). The reasons for the Pluronic – titania based films having the highest efficiency are that (i) the films are thicker than the others, which results in an increase of the UV absorbance, and that (ii) the grains are smaller and exhibit a well-structured surface, which gives in higher photochemical reaction rates because of the increased active surface area. An appropriate balance between the two factors should be achieved to obtain highest photoefficiency. First, while films have to be sufficiently thick to achieve sufficient catalyst loading, opacity and internal mass transfer resistance is also increased with increased thickness. This could lead to higher electron/hole recombination rates. Aggregation and growth of TiO2 particles in the interior region of thick films, which underwent longer calcinations times, may also cause a decrease in the number of surface active sites [12]. Second, as far as the optimal grain size is concerned, smaller grains result in a higher active surface area, but electron-hole recombination rates are also higher for the smaller particles. 4. Summary Nanostructured and transparent titania films with anatase grain sizes of as small as 10 nm were deposited on SiO2 precoated soda – lime glass substrates in order to enhance their activity for the photocatalytic degradation of an azo-dye (Plasmocorinth B). The use of the Pluronic F-127 templating agent in a surfactant-assisted sol –gel process reduced the number of dip coatings necessary to obtain a sufficient titania loading with photocatalytically active surface area. The films were immersed in the colored solution and photobleaching was followed in situ to near completion in one hour. The titania catalyst can be easily removed from the solution, which is one of the principal advantages of using the immobilized films as catalysts rather than titania powders. Acknowledgements
Fig. 5. Photobleaching of Plasmocorinth B solution (absorbance at 527 nm vs. irradiation time) in the absence and presence of TiO2 films deposited from various sols on SiO2-glass substrates with 4 dipping – heating cycles.
Authors wish to thank Prof. B. Orel and Prof. V. Kau*i* for valuable discussions and XRD measurements, respectively, Dr. M. Cˇekada for thickness measurements, D. Strm*nik and M. Zorko for SEM and Dr. K. Dahmouche for BET surface
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