Journal of Membrane Science 218 (2003) 269–277
Preparation TiO2 /Al2 O3 composite hollow fibre membranes Shaomin Liu a , K. Li b,∗ a
Institute of Environmental Science and Engineering, 18 Nanyang Drive, Singapore 637723, Singapore b Department of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, UK Received 8 October 2002; accepted 12 April 2003
Abstract Aluminium oxide (Al2 O3 ) hollow fibres were prepared by a combined phase inversion/sintering method. An organic binder solution (dope) containing suspended Al2 O3 powders is spun into a hollow fibre precursor, which is then sintered at elevated temperatures. In spinning the hollow fibre precursor, polyethersulfone (PESf), N-methyl-2-pyrrolidone (NMP) and polyvinylpyrrolidone (PVP) were used as a polymer binder, a solvent and an additive, respectively. The prepared Al2 O3 hollow fibre membranes with suitable surface roughness were then used as substrates for the fabrication of porous or dense TiO2 /Al2 O3 composite membranes via direct deposition using an aqueous solution containing titanium tetrafluoride. The prepared Al2 O3 substrates and the TiO2 /Al2 O3 composite hollow fibre membranes were characterized using scanning electron microscope (SEM), X-ray diffraction (XRD) and gas permeation techniques. The results indicate that TiO2 -based hollow fibre membranes, consisting of small anatase nano-particles, exhibit excellent adhesion to the outside surface of the tailor-made Al2 O3 hollow fibre substrates. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Hollow fibres; Ceramic membranes; Al2 O3 ; TiO2
1. Introduction Titania membranes have received significant attention in recent years due to their unique characteristics such as high water flux, semiconducting properties, photocatalysis, and chemical resistances over other membrane materials such as ␥-alumina, silica, and zirconia [1–5]. The potential applications of titania membranes are numerous such as in ultrafiltration processes and in catalytic/photocatalytic membrane reactors for liquid and gas separations/reactions. Currently, TiO2 membranes have been mainly prepared via conventional tape casting processes. Chem∗ Corresponding author. Tel.: +44-1225-386372; fax: +44-1225-386894. E-mail address:
[email protected] (K. Li).
ical vapour deposition (CVD), sputtering, and sol–gel methods are also applicable for the fabrication of homogeneous TiO2 membranes. In these methods, however, strict processing conditions are commonly required for high crystallinity of the ceramic membranes, which is very cumbersome and time consuming, and are often not economically viable. In addition, the TiO2 -based membranes studied so far are in the form of finite sized tubes with diameters of at least several millimetres or flat discs and consequently have low surface area/volume ratios (typically a maximum of 100 m2 · m−3 ). These low area/volume ratios compare unfavourably with polymeric hollow fibre modules where area/volume ratios of several thousand are obtainable; this limits the application of current inorganic tubular and disc membranes. This limitation is most evident in catalytic membrane
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00184-4
270
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
reactors, where it is desirable to maximize the area of the membrane module to increase the permeation rate to remove the product species from the reaction zone. There are a few methods available for preparing inorganic hollow fibres, including dry spinning a system of inorganic material and binder [6], wet spinning a suitable inorganic material-containing solution [7], depositing fibres from the gas phase on to a substrate, or pyrolyzing the polymers [8–10]. Recently, the well-known phase inversion method, commonly employed for spinning polymeric hollow fibre membranes, have been successfully adopted in preparing the inorganic aluminium oxide (Al2 O3 ) hollow fibres [11–16]. Because of the phase inversion characteristics, the prepared inorganic hollow fibres possess an asymmetric structure, which provides a better permeability for a given thickness. Thus, they can be used not only in many separation processes, but also to be served as a porous support for composite membrane formation [17]. The objective of this study is to investigate the possibility of preparation of TiO2 /Al2 O3 composite hollow fibre membranes. The Al2 O3 asymmetric hollow fibre membranes used as a support were first fabricated using a combined phase inversion/sintering method, followed by a new and simple deposition technique using an aqueous solution of titanium tetrafluoride to obtain a thin and porous or dense TiO2 layer on the Al2 O3 hollow fibre support. 2. Experimental 2.1. Materials Commercially available aluminium oxide powders with two different particle diameters of 0.3 m (gamma/alpha, surface area 15 m2 g−1 ) and 1 m (alpha, surface area 10 m2 g−1 ) [purchased from Alfa AESAR, A Johnson Matthey company] were used as membrane substrate materials. Polyethersulfone (PESf) [Radel A-300, Ameco Performance, USA], and N-methyl-2-pyrrolidone (NMP) [Synthesis Grade, Merck] were used for preparing the starting solution. Polyvinylpyrrolidone (PVP, K90) [GAF® ISP Technologies Inc., Mw = 630,000] was used as an additive. Tap water was used as both the internal and external coagulants. Hydrochloric acid (HCl), aque-
ous ammonia (NH4 OH), deionized water and titanium tetrafluoride (TiF4 , Aldrich) were used for preparing the coating solution for TiO2 -based composite membrane formation. 2.2. Preparation of the aluminium oxide (Al2 O3 ) hollow fibre support The required quantity of NMP was taken in 1 l wide-neck reaction flask and the PESf was slowly added over a period of 30 min to form the polymer solution. After the polymer solution was formed, a given amount of aluminium oxide (0.3 and 1 m or a mixture of them) was then added into the polymer solution slowly, while Heidolph RZR 2000 stirrer was used at a speed of 314.16 × 10−1 rad s−1 (∼300 rpm) to ensure that all the aluminium oxide powders is dispersed uniformly in the polymer solution. PVP as an additive was also introduced into the solution to modulate its viscosity. Finally, the polymer solution was degassed at the room temperature. The degassed starting (dope) solution containing the dispersed aluminium oxide powders was transferred to a stainless steel reservoir and pressurized to 1–3 bar using nitrogen. A tube-in-orifice spinneret with orifice diameter/inner diameter of the tube of 2.0/0.72 (mm) was used to obtain hollow fibre precursors. The air-gap was kept at 2 cm for all spinning runs. Finally, the forming hollow fibre precursor was passed through a water bath to complete solidification process and thoroughly washed in water. The details of the spinning equipment and procedure on hollow fibre spinning have been described elsewhere [18]. The formed hollow fibre precursors were first heated in a CARBOLITE furnace at about 500 ◦ C for 2 h to remove the organic polymer binder and then were calcined at a high temperature for about 10 h to allow the fusion and bonding to occur. The calcination temperature used in this study was between 1300 and 1600 ◦ C. 2.3. Formation of TiO2 /Al2 O3 composite hollow fibre membranes Hydrochloric acid (HCl) and aqueous ammonia (NH4 OH) were used to adjust pH of deionized water (1.0 l) to 2.1. Titanium tetrafluoride (TiF4 , Aldrich) was then dissolved in this solution to give a concentration of 0.04 M, during which pH was changed to
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
1.6. Al2 O3 hollow fibres with the length of 35 cm were immersed in the solution and maintained at 60 ◦ C for 1–4 days. Using these conditions, anatase TiO2 membranes were successfully deposited on the outside surface of alumina fibres. The prepared composite membranes were washed with deionized water and dried at 60–100 ◦ C for 24–48 h. A few repetitions of the deposition and drying were carried out to obtain a dense TiO2 membrane.
271
method. The basic preparation conditions and physical properties of the resulting hollow fibres are given in Table 1. The hollow fibre prepared from 0.3 m particles exhibits appropriate surface roughness and was selected as the substrate for the further preparation of TiO2 /Al2 O3 composite hollow fibre membranes. 3.1. Morphology and physical properties of the Al2 O3 hollow fibre support
2.4. Membrane characterizations The gas permeation rate was measured at room temperature using soap-bubble meter with a nitrogen flux under 1.01325 × 105 Pa (1 atm) gas pressure difference. Crystalline phase of TiO2 was determined by X-ray diffraction method (XRD, Shimadzu XRD-6000, Cu K␣ radiation). Structures of the prepared TiO2 membranes and Al2 O3 hollow fibres were examined using scanning electron microscope (SEM, JEOL JSM-5600). Mechanical strength of the hollow fibre was measured using a tensile tester (Instron Model 5544). The details of the strength characterization procedure have been described elsewhere [13].
3. Result and discussion In this study, 14 batches of Al2 O3 hollow fibres were prepared using a combined phase inversion/sintering
3.1.1. Morphology study SEM micrographs of the hollow fibre precursor and its sintered fibre, spun from the dope containing Al2 O3 (0.3 m) 50%, PESf 10%, PVP 0.5% and NMP 39.5%, and coagulated in water bath at room temperature, were shown in Figs. 1 and 2, respectively. It can be seen from the micrograph of Fig. 1a that the o.d. and i.d. of the fibre precursor prepared were measured to be 1705 and 1118 m, respectively. The micrograph of Fig. 1b illustrates that near the inner wall of the fibre precursor, long finger-like structures are present and that at centre and outer wall of the hollow fibre precursor, sponge-like structures are possessed. The appearance of the fibre cross-sectional structures shown in Fig. 1b can be attributed to the general phase separation phenomena between the polymer binder and the coagulant. Micrograph of Fig. 1c shows the surface of the hollow fibre precursor where the Al2 O3 particles can vividly seen.
Table 1 Experimental results Batch no.
Al2 O3 /PESf (weight ratio)
Al2 O3 powders 1/0.3 (m)
Sintering temperature (◦ C)
Gas permeability (mol m−2 Pa−1 s−1 ) (N2 , 1.01325 × 105 Pa (1 atm))
Bending strength σ F (MPa)
1 2 AI BI CI AII BII CII AIII BIII CIII 3 4 5
5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 5:1 8:1 8:1 10:1
0/100 0/100 0/100 50/50/0 100/0/0 0/100 50/50/0 100/0/0 0/100 50/50/0 100/0/0 100/0/0 100/0/0 100/0/0
1300 1450 1500 1500 1500 1550 1550 1550 1600 1600 1600 1500 1550 1550
– 4.93 4.53 3.90 6.50 2.54 3.59 3.75 0.95 0.98 1.15 6.03 3.41 4.44
20.9 45.9 51.3 59.3 48.6 80.9 93.56 76.1 246.16 253.08 182.41 196.64 251 304.2
× × × × × × × × × × × × ×
10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−6 10−7 10−8
272
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
Fig. 1. SEM diagrams of the hollow fibre precursor prepared from 0.3 m Al2 O3 particles: (a) cross-section; (b) membrane wall; (c) membrane surface.
Fig. 2 shows the micrographs of the sintered hollow fibre. The sintering process was carried out in air at temperature of 1500 ◦ C. It can be seen from the micrograph of Fig. 2a that the o.d. and i.d. of the sintered fibre were shrunk from 1705 and 1118 to 1333 and 911 m, respectively. The cross-sectional structure of
Fig. 2. SEM diagrams of the sintered hollow fibre prepared from 0.3 m Al2 O3 particles: (a) cross-section; (b) membrane wall; (c) membrane surface.
the sintered fibre as shown in Fig. 2b is the same as that of the precursor, i.e. the sponge-like structures are at centre and outer wall, while the long finger-like structures are located at the inner wall of the fibre. Further comparing the SEM photos, especially for the
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
surfaces between the precursor (Fig. 1c) and the sintered fibre (Fig. 2c) reveals that the pore quantity and pore size may have changed after the sintering process although the general structure is maintained. Such structure changes depend on the composition of the dope solution and the sintering temperature, which are discussed and presented in the following section. 3.1.2. Mechanical strength and permeation property Experimental data on the hollow fibre mechanical strength and permeation property are shown in Figs. 3 and 4. In Fig. 3, the hollow fibre mechanical strength and gas permeability are plotted against the sintering temperature. The data were obtained from the hollow fibre spun from Al2 O3 /PESF weight ratio of 5. It can be seen that increase of sintering temperature would enhance the mechanical strength. For example, at temperatures of 1300 and 1550 ◦ C sintered for 10 h, the three-point (3P) values are 20.9 and 80.9 MPa (also shown in Table 1), respectively. When the sintering temperature is less than 1500 ◦ C, the 3P value is proportional to the sintering temperature. When the
273
sintering temperature is greater than 1550 ◦ C, the 3P value increases sharply as the sintering temperature is further increased. The results indicate that sintering temperature should be chosen at around three-fourth of the material’s melting point. Otherwise, the ceramic particles would not sufficiently fuse or bond during sintering and the mechanical strength is low. In the case of Al2 O3 , the melting point is 2054 ◦ C and therefore the sintering temperature should be above 1500 ◦ C. It, thus, suggests that preparation of the Al2 O3 hollow fibre membranes with high mechanical strength are possible at the sintering temperature of 1533 ◦ C or higher, as a slight increase in temperature would dramatically increase the mechanical strength. Sintering of the hollow fibre at the temperature of 1600 ◦ C shows, of course, the increase in the mechanical strength, however, gas permeability is decreased considerably as shown in Fig. 3. Therefore, there is a trade-off between the mechanical strength and gas permeability. The 3P values at various sintering temperatures suggest that the sintering temperature at 1550 ◦ C would give sufficient strength
Fig. 3. Mechanical strength and gas permeability of the hollow fibre membranes vs. sintering temperatures (prepared from 0.3 m Al2 O3 powder at Al2 O3 /PESf ratio of 5; sintered for 10 h).
274
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
Fig. 4. Mechanical strength and gas permeability of the hollow fibre membranes prepared at different Al2 O3 content (1.0 m Al2 O3 particles; sintering temperature of 1550 ◦ C).
for the fibre to be fabricated into a module without breaking. The hollow fibre precursor formed through the phase inversion techniques contains the Al2 O3 powder and the PESf binder. During the sintering process, the PESf is removed and the Al2 O3 hollow fibre is ultimately formed. Therefore, the Al2 O3 content in the spinning dopes plays the important roles in determining its mechanical strength. Fig. 4, where the hollow fibre samples were sintered at the same temperature of 1550 ◦ C, illustrates the effect of Al2 O3 content on the fibre mechanical strength and its gas permeability. It can be seen that the 3P value enhances greatly, as the Al2 O3 /PESf ratio is increased. Compared to the sintering temperature, the increase of the Al2 O3 powder content in the spinning dope would result in a much obvious effect on the fibre mechanical strength. It is, therefore, follows that in order to produce a Al2 O3 hollow fibre membrane with higher mechanical strength, the higher Al2 O3 content in the solution
dope must be maintained. At Al2 O3 /PESf ratio of 7 or grater, reduction in gas permeability is tailed, indicating that the membrane is transformed to a much denser structure. 3.1.3. Membrane surface roughness The above developed inorganic hollow membranes can be used not only in microfiltration and ultrafiltration applications at high temperatures, but also be served as porous supports for composite membrane formation. The roughness of the prepared hollow fibre substrate may decide the minimum achievable coating layer thickness. The surface roughness of the sintered membrane substrate is related to many factors such as the Al2 O3 content in the dope, the particle size of the starting powder, sintering temperature, sintering time, and so on. In general, the hollow fibres fabricated from a lower Al2 O3 content in the dope, a smaller size in the starting powder, a higher sintering temperature, and a longer sintering time show a smoother surface.
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
Due to the lack of instrument to measure the surface roughness, quantitative data of hollow fibre fabricated at different conditions is not available. However, our experiment results reveal that hollow fibre membranes fabricated from 100% of 0.3 m particle sized starting powders (batch AII in Table 1) have shown a smoother outside surface compared to the hollow fibres prepared from other conditions. Therefore, in this study, Al2 O3 hollow fibre prepared from particle di-
275
ameters of 0.3 m has been chosen as the substrate for the TiO2 /Al2 O3 composite hollow fibre membrane preparation and the results are given below. 3.2. Formation of TiO2 /Al2 O3 composite hollow fibre membranes Direct deposition techniques in supersaturated chemical solutions to prepare metal oxide films, such
Fig. 5. SEM pictures of the TiO2 /Al2 O3 composite hollow fibre membranes with different deposition time (a, b, 17 h; c, d, 48 h; e, f, 96 h; a, c, e: surface view; b, d, f: cross-section view; samples are dried at 60 ◦ C for 24 h).
276
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
as, SiO2 [19], SnO2 [20], FeOOH [21], V2 O5 [22], and TiO2 [23,24] have been recently developed. In particular, thin films of well-crystallized anatase titania particles are easily obtained from aqueous solutions of titanium tetrafluoride [24] at relatively low temperatures (i.e. 60 ◦ C). The chemical reactions of TiF4 hydrolysis in formation of TiO2 have been understood, which take place as in the following steps [24]: TiF4 → Ti(OH)4−x Fx → TiO2 Using this technique, TiO2 layers can be deposited on various substrates with complex shapes through heterogeneous nucleation. Based on the same principle, TiO2 /Al2 O3 composite hollow fibre membranes have been prepared in this study. Fig. 5 (SEMs) shows the growth of TiO2 layers coated on the outside surface of the hollow fibre substrate with different deposition time. Comparison of microstructures of the Al2 O3 hollow fibre membranes before and after deposition, the growth of TiO2 membrane is very clear, particularly for the samples with a deposition time of 96 h (Fig. 5e and f). Fig. 5 also illustrates that the thickness of TiO2 layer increased with the deposition time. A X-ray diffraction pattern for the deposited membranes on the Al2 O3 substrate indicated that the deposited membranes consisted of anatase particles with high crystallinity. As shown in Fig. 6, before deposition, the crystalline phase of the
Table 2 Gas permeability changes with the number of coating times Number of coating
Deposition time (h)
Drying temperature (◦ C)
Gas permeability (mol m−2 Pa−1 s−1 ) (N2 )
1 2 3 4
96 59 48 48
100 100 100 100
11.7 × 107 3.73 × 107 0.11 × 107 0
fibre belongs to ␣-alumina (Fig. 6b) and after deposition, the small particles forming the coating layer are identified as anatase TiO2 . Due to the hydrophilic properties of the prepared Al2 O3 hollow fibres, the formed TiO2 membranes showed excellent adhesion to the hollow fibre substrate surface. As suggested by Shimizu et al. [24], the TiO2 growth initially occurred at the surface of the substrate and the pores between the Al2 O3 grains in the hollow fibre surface were gradually filled up by TiO2 particles during the deposition. Therefore, the gas permeability of the deposited hollow fibre membranes would become smaller comparing to that of the original substrate. A few repetitions of the deposition and drying procedure for the same hollow fibre have turned the membrane from porous to almost gas-tight membrane as shown in Table 2.
4. Conclusions
Fig. 6. XRD patterns for (a) Al2 O3 and anatase TiO2 coatings ((∗) anatase TiO2 ); and (b) Al2 O3 .
Asymmetric Al2 O3 hollow fibres with suitable surface roughness have been prepared using a combined phase inversion/sintering technique and employed as a substrate for the fabrication of porous or dense TiO2 /Al2 O3 composite membranes, which were obtained directly by a simple deposition technique using an aqueous solution containing titanium tetrafluoride. The deposited TiO2 layer, consisting of small anatase TiO2 nano-particles with high crystallinity, exhibit excellent adhesion to the Al2 O3 support surface. In this research work, Al2 O3 hollow fibre substrates with o.d. of 1.3 mm and length of 35 cm have been successfully deposited with porous or gas-tight TiO2 membranes based on the different coating times. This simple direct deposition method is effective in the
S. Liu, K. Li / Journal of Membrane Science 218 (2003) 269–277
preparation of supported TiO2 membranes and could be scaled to large applications.
Acknowledgements The authors gratefully acknowledge the research funding provided by EPSRC in the United Kingdom (grant no. GR/N38640). References [1] A. Larbot, J. Fabre, C. Guizard, L. Cot, J. Gillot, New inorganic ultrafiltration membranes: titania and zirconia membranes, J. Am. Ceram. Soc. 72 (2) (1989) 257. [2] S.H. Hyun, B.S. Kang, Synthesis of titania composite membranes by the pressurized Sol–Gel Technique, J. Am. Ceram. Soc. 79 (1) (1996) 279. [3] L. Wu, N. Xu, J. Shi, Novel method for preparing palladium membranes by photocatalytic deposition, AIChE J. 46 (5) (2000) 1075. [4] A. Voigt, P. Puhlfurß, J. Topfer, Preparation and characterization of microporous TiO2 -membranes, Key Eng. Mater. 132–136 (1997) 1735. [5] P. Puhlfurß, A. Voigt, R. Weber, M. Morbe, Microporous TiO2 -membarnes with a cut off <500 Da, J. Membr. Sci. 174 (2000) 123. [6] R.A. Terpstra, J.P.G.M. Van Eijk, F.K. Feenstra, Method for the production of ceramic hollow fibres, US Patent 5,707,584 (1998). [7] K.H. Lee, Y.M. Kim, Asymmetric hollow inorganic membranes, Key Eng. Mater. 61–62 (1991) 17. [8] J.E. Koresh, A. Sofer, Molecular sieve carbon permselective membrane. Part I. Presentation of a new device for gas mixture separation, Sep. Sci. Technol. 18 (1983) 723. [9] J.E. Koresh, A. Sofer, Mechanism of permeation through molecular-sieve carbon membrane, J. Chem. Soc., Faraday Trans. 82 (1986) 2057. [10] V.M. Linkov, R.D. Sanderson, E.P. Jacobs, Highly asymmetrical carbon membranes, in: Proceedings of the 34th IUPAC Symposium, Prague, Czechoslovakia, 13–18 July 1992, Macromolecules 7 (1992) 56.
277
[11] X. Tan, S. Liu, K. Li, Preparation and characterization of inorganic hollow fibre membranes, J. Membr. Sci. 188 (2001) 87. [12] J. Luyten, A. Buekenhoudt, W. Adriansens, J. Cooymans, H. Weyten, F. Servaes, R. Leysen, Preparation of LaSrCoFeO3−x membranes, J. Membr. Sci. 135 (2000) 637. [13] S. Liu, X. Tan, K. Li, R. Hughes, Preparation and characterisation of SrCe0.95 Yb0.05 O2.975 hollow fiber membranes, J. Membr. Sci. 193 (2001) 249. [14] C. Chaumette, N. Dinges, U. Herterich, N. Stroh, Ceramic hollow fiber membranes, in: Proceedings of the ICOM by the European Membrane Society, Toulouse, Frankreich, 2002, Poster presentation. [15] N. Stroh, N. Dinges, A. Goldbach, ␣-Al2 O3 hollow fiber membranes, in: Proceedings of the 7th ICIM, Dalian, China, 2002, Oral presentation. [16] Shaomin Liu, K. Li, R Hughes, Preparation of porous aluminium oxide (Al2 O3 ) hollow fibre membranes by a combined phase-inversion and sintering method, Ceram. Int., in press. [17] J.J. Hammel, Porous inorganic siliceous-containing gas enriching material and process of manufacture and use, US Patent 4,853,001 (1989). [18] S.P. Deshmukh, K. Li, Effect of ethanol composition in water coagulation bath on morphology of PVDF hollow fibre membranes, J. Membr. Sci. 150 (1998) 75. [19] H. Nagayama, H. Honda, H. Kawahara, A new process for silica coating, J. Electrochem. Soc. 135 (1988) 2013. [20] K. Tsukuma, T. Akiyama, H. Imai, Liquid phase deposition film of tin oxide, J. Non-Cryst. Solids 210 (1) (1997) 48. [21] S. Deki, Y. Aoi, J. Okibe, H. Yanagimoyo, A. Kajinami, M. Mizuhata, Preparation and characterization of iron oxyhydroxide and iron oxide thin films by liquid-phase deposition, J. Mater. Chem. 7 (9) (1997) 1769. [22] S. Deki, Y. Aoi, A. Kajinami, A novel wet process for the preparation of vanadium dioxide thin film, J. Mater. Sci. 32 (16) (1997) 4269. [23] S. Deki, Y. Aoi, O. Hiroi, A. Kajinami, Titanium(IV) oxide thin films prepared from aqueous solution, Chem. Lett. 6 (1996) 433. [24] K. Shimizu, H. Imai, H. Hirashima, K. Tsukuma, Low-temperature synthesis of anatase thin films on glass and organic substrates by direct deposition from aqueous solutions, Thin Solid Films 351 (1999) 220.