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Fabrication of macroporous titanium dioxide film using PMMA microspheres as template
Journal of Colloid and Interface Science 386 (2012) 73–79
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Fabrication of macroporous titanium dioxide film using PMMA microspheres as template Hongzhong Zhang a,⇑, Feng Dong a, Shaoming Fang a, Changming Ye a, Minghua Wang a, Haijun Cheng a, Zhouxiang Han a, Shengnan Zhai b a b
Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, People’s Republic of China University of Wollongong, Wollongong, New South Wales 2522, Australia
a r t i c l e
i n f o
Article history: Received 20 April 2012 Accepted 25 July 2012 Available online 3 August 2012 Keywords: PMMA microspheres Template Titanium dioxide Composite materials Thin films
a b s t r a c t A novel synthetic procedure is described for the fabrication of macroporous titanium dioxide (TiO2) films with an ordered, uniform pore framework comprised of nanocrystalline anatase mainly. The synthetic approach involved several processes. First, polymethyl methacrylate (PMMA) microspheres (87 nm) were synthesized by using a dispersion polymerization technique in the presence of Fenton reagent (FeSO4/ H2O2) as a novel initiator, which has advantages such as simple and fast polymerization process without deoxygenation. Next, the templates of PMMA microspheres were assembled on clean substrates by dipdrawing technique. Finally, the macroporous TiO2 films with the average size of pores about 87 nm were obtained by sol-dipping template method and calcination to remove the templates at 550 °C. The test results of X-ray diffraction indicate that the nanocrystalline of anatase formed after calcination. The mechanisms of PMMA polymerization and template formation were proposed. Furthermore, both structures and morphologies of the composite films were investigated with field emission scanning electron microscope, and the processes of the thermal decomposition of PMMA and TiO2 gel were also discussed with thermo gravimetric analysis. This ordered and uniform pore framework could be used as the promising ultrafilter membranes showing active photocatalysis without intensive fouling. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction In the coming decades, one of the most extensive problems afflicting people throughout the world is inadequate access to clean water and sanitation. However, water pollution has been becoming a more and more serious problem with the development of industries. Fortunately, recent activities in water treatment research offers hope in mitigating the impact of impaired waters around the world [1]. For example, membranes represent promising alternatives to conventional water and waste water treatment processes because membrane based separations are energy efficient and cost effective when optimized [2]. No matter which type of membranes, however, a major obstacle to further use in industrial operations is flux decline resulting from fouling [3]. Several types of fouling can occur in the membrane system, for example, crystalline fouling, organic fouling, particulate and colloidal fouling, and microbial fouling [4]. Several strategies to alleviate the membrane fouling have therefore been investigated. Various methods to reduce fouling have been carried out, which generally involve pretreatment of the feed solution, modification of the ⇑ Corresponding author. E-mail address: [email protected] (H. Zhang). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.07.080
membrane surface properties, optimization of module arrangement and process conditions, and periodic cleaning. Even after long periods of the developments, biofouling caused by microorganisms and biofilm accumulation on membrane still remains the main reason for flux decline [5]. Comparing with the conventional polymeric membranes, ceramic membranes are playing an important role in water treatment due to their many advantages, such as high temperature stability, high pressure resistance, good chemical stability, high mechanical resistance, and long life [2]. Alumina, titania, and zirconia are considered as the most common porous ceramic membrane materials. Among ceramic membranes, titanium dioxide (TiO2) has been the focus of intensive investigations in recent years, because of its photocatalytic effect that can decompose organic chemicals and destroy bacteria during the photocatalytic process illuminated semiconductor absorbs light and generates active species [6]. In general, the performance of membranes is attributed to two main factors: pore size distribution and porosity. For any one medium with specific size in water, more uniform in terms of membrane pores generate higher retention rate, higher density of pores, and more permeation efficiency. Therefore, the permeability and antifouling ability of TiO2 membranes with well-controlled and uniform pores can be significantly improved. Although porous TiO2
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membranes can be fabricated via nanoparticle sintering, as Paulose [7] pointed out, these membranes do not possess uniform size and shape of pores. Ultrathin nanoporous TiO2 membranes can be produced by preparing a self-supporting polymer–titania composite film on a nanoporous alumina substrate and then removing the polymer using chemical methods or plasma treatment [8]. However, these membranes are difficult to handle and the polymer removal disturbs the size and shape of pores. Crystalline mesoporous TiO2 membranes with well-defined size and shape of pores can be fabricated via surfactant templating, but are generally not strong enough for normal handling [7]. Over the past decades, polymer microspheres showing uniform size and large surface area have been often used as the templates to fabricate ordered macro-mesoporous materials/films [9–14]. In many cases, the performance of these polymer microspheres significantly depends on their size, shape, dispersity, and functional surface. For example, polystyrene (PS) microspheres have been mainly used as various templates to form ordered macroporous materials/films due to their good mono-dispersivity and controllability of size [13,15–18]. However, the materials or membranes with highly ordered meso-microporous structures using PS microspheres as templates have not been produced yet, because of their large size (beyond 500 nm generally) and poor functional surface [19]. Polymethylmethacrylate (PMMA) microspheres are therefore perfect alternative template associated with not only their functional surface, also easy removal of templates without any fracture of ordered porous frameworks [11]. However, until recently, only a limited number of results about templating ordered porous structures using PMMA microspheres have been reported. For example, Sasahara et al. [11] have used PMMA microspheres as a template to fabricate the macroporous ceramic thick films consisting of nanosized particles. In this case, the PMMA microspheres with a nominal uniform particle size of 800 nm were purchased commercially without providing any information of PMMA polymerization. In another case [20], although PMMA microspheres were prepared by in situ thermal polymerization, too long time (72 h) of synthesis and poor mono-dispersivity of microsphere size are the main defects in comparing with PS microspheres. In this work, we introduce a novel, reproducible synthetic method for preparing macroporous TiO2 films with ordered framework by using the microspheres-assembled template of organic polymer. Herein, non-crosslinked, monodisperse PMMA microspheres were fabricated via a dispersion polymerization method in the presence of Fenton reagent (FeSO4/H2O2) as an initiator. We demonstrate that the PMMA microspheres obtained are perfect with uniform size about 87 nm and the size deviation is less than 5%. Comparing with the conventional emulsion polymerization, this facile synthesis technique exhibits some advantages such as simple and quick polymerization process without deoxygenation due to using a novel initiator that has not been reported. Moreover, the TiO2 precursor solution prepared by sol–gel method was then infiltrated into the interstices among the PMMA templates by using sol-dipping method, followed by calcination to remove the template and produce the ordered macroporous TiO2 films. We also believe that the obtained ordered framework is promising to be used as the ultrafiltration membrane showing active photocatalysis without intensive fouling.
2. Materials and methods 2.1. Materials In the preparation of PMMA microspheres templates and TiO2 sol, the following materials were used: methyl methacrylate (MMA, CP, distillated before experiment) and polyvinylpyrrolidone
(PVP, K-30) were purchased from Sinopharm Chemical Reagent Co. Ltd. Ferrous sulfate (FeSO4, GR), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, AR), acetone (AR), glacial acetic acid (HAc, AR), diethyl ether (CH3CH2OCH2CH3, AR), and ammonia (NH3H2O, 25%) were obtained from Tianjin Kermel Chemical Reagent Co. Ltd. Tetrabutyl titanate (TBOT, CP) was purchased from Shanghai SSS Reagent Co. Ltd. Triethanolamine (TEA, AR) was purchased from Tianjin FuYu Fine Chemicals Co. Ltd. Absolute ethyl alcohol (EtOH, AR) was purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co. Ltd. 2.2. Polymerization of monodispersed PMMA microspheres Non-crosslinked, monodispersed PMMA microspheres were synthesized by using a dispersion polymerization technique. First, dispersion medium constituted by deionized water (30 mL) and diethyl ether (10 mL) with a certain proportion was heated at 70 °C in water bath, followed by 0.40 g PVP was added in the dispersion medium to be dissolved under magnetic stirring. Next, a certain amount of MMA purified for wiping off the inhibitor was added in the mixed solution, followed by the addition of HAc to adjust pH value at 3.0–4.0. Then, Fenton reagent used as an initiator, that is, 0.005 g FeSO4 and 100 lL H2O2, was also added in the mixed solution. Finally, the mixed solution was stirred for 8 h at 70 °C with stirring speed of 300 rpm. The resulting PMMA microspheres were remained suspended in the mother liquor until needed. 2.3. Assembly of the PMMA microspheres templates The PMMA microspheres templates were assembled on the substrates of monocrystalline silicon piece by the dip-drawing technique to observe the porosity and morphologies of these templates. Firstly, the substrates (1 3 cm) were cleaned ultrasonically for 30 min in the solution mixed with H2O2 (30%), NH3H2O, and deionized water with a certain proportion of 1:1:2 (volume ratio). Then, the clean and dried substrates were immersed vertically in the emulsion of PMMA for 5 min. Finally, the substrates were slowly drawn out (drawing speed of 0.1 cm/min) and dried at 30 °C for 15 min. After the procedure was performed as mentioned above, opalescent PMMA templates with prismatic colors like rainbow were obtained, depending on the angle of observation. 2.4. Preparation of TiO2 Sol TiO2 precursor solution was prepared by sol–gel method at room temperature according to the following procedure: TBOT was dissolved in the mixture solution of 20 mL EtOH and 3 mL TEA under vigorous stirring and kept for 2 h. Thus, the homogeneous, transparent, and stabilized TiO2 sol with yellowish green color was obtained. 2.5. Preparation of macroporous TiO2 films Sol-dipping template method was used to fill TiO2 sol into the interstices among the PMMA templates. First, the substrates of monocrystalline silicon with PMMA array template on it were immersed in TiO2 sol and kept for 1 min. Then, the substrates were drawn out from sol and dried at room temperature for 3 h to form the composite structure of thin film covered by PMMA microspheres. Finally, the resulting macroporous TiO2 films were obtained by calcination to remove the PMMA templates as the following recipe: calcination of the films was carried out in air with a heating rate of 2.5 °C/min starting from room temperature to 220 °C, maintained it at 220 °C for 1 h; 1 °C/min from 220 °C to 430 °C, maintained it at 430 °C for 1 h; 5 °C/min from 430 °C to
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550 °C and maintained it at 550 °C for 4 h, respectively. After calcination, the samples were cooled to room temperature in the furnace.
1272.2
1242.5
100
The IR spectrum of the PMMA synthesized by the recipe of Section 2.2 was recorded with a fourier transform infrared (FT-IR) spectrophotometer (Tensor27, Bruker). The diameter of PMMA microsphere was estimated using transmission electron microscopy (TEM, JSM-2001). Morphologies of the templates and macroporous films were examined using field emission scanning electron microscope (FESEM, JSM-7001F). X-ray diffraction (XRD) patterns of TiO2 powders were recorded on a Bruker D8 Advance X-ray diffractometer operated at 40 kV and 35 mA using Cu Ka radiation and nickel filter (k = 0.15406 nm). The thermal-decomposition behavior of the PMMA template filled with TiO2 precursor was measured using a thermogravimetric and differential thermal analysis (TG/DTA) instrument (Diamond, USA).
Transmittance/%
80
2.6. Characterization
988.5
60
843.3
2986.4 2953.1
40
1386.4
3452.4
1443.2 1193.1
20
1149.0 1731.6
0 4000
3500
3000
2500
2000
1500
1000
500
0
Wavenumber/cm-1 Fig. 2. FT-IR spectrum of PMMA sample.
Table 1 The recipes for polymerizing PMMA microspheres.
3. Results and discussion Fenton reagent is a redox initiation system consisting of Fe2+ and H2O2, which is extensively used in the oxidation treatment of waste water. The chain reaction between Fe2+ and H2O2 in acidic medium can catalyze OH free radical in which the oxidation potential is +2.8 V. The reaction process [21] is explained as follows:
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð1Þ
Fe2þ þ OH ! Fe3þ þ OH
ð2Þ
H2 O2 þ OH ! H2 O þ OOH
ð3Þ
Fe2þ þ OOH ! Fe3þ þ OOH
ð4Þ
Fe3þ þ OOH ! Fe2þ þ Hþ þ O2
ð5Þ
The emulsion of homogeneous system in this work is composed of monomer (MMA), initiator (Fenton reagent), stabilizing agent (PVP), and dispersion medium (H2O and diethyl ether). When a given temperature is reached, the polymerization of AC@CA double bond in MMA could be carried out by OH caused by Fenton reagent. As the molecular weight of PMMA increases, the solubility of polymer decreases. Thus, when the chain length of PMMA is increased to a critical value, the polymer should be separated and precipitated from the dispersion medium. Then, the primary nuclear shells of
Sample MMA PVP no. (mL) (g)
H2O Diethyl Reaction Temperature FeSO4 H2O2 time (h) (°C) (mL) ether (g) (lL) (mL)
a b c d e f g h
30 30 30 30 30 30 30 30
4 6 8 4 4 4 4 4
0.40 0.40 0.40 0.20 0.40 0.40 0.40 0.40
10 10 10 10 10 10 10 10
8 8 8 8 6 7 8 8
70 70 70 70 70 70 60 70
0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.010
100 100 100 100 100 100 100 200
polymer are generated by PVP and held in the emulsion. Consequently, the volume of nuclear shell is increased with the ongoing polymerization because there are also MMA molecules and OH in the primary nuclear shell. Therefore, the microspheres of PMMA will be formed. Fig. 1a and b shows the images of transmission electron microscopy (TEM) and field emission scanning electron microscope (FESEM) of PMMA microspheres prepared in this work, respectively. It can be seen that the average diameter size of the PMMA microspheres obtained is about 87 nm, and the size deviation is less than 5%. The FT-IR spectrum of the polymer sample obtained is shown in Fig. 2. The band at 1731.6 cm1 is attributed to symmetric anhydride C@O stretching modes of the methacrylate group, and the
Fig. 1. TEM image (a) and FESEM image (b) of PMMA microspheres.
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Fig. 3. FESEM images of PMMA microspheres with different polymerization recipes.
bands at 1193.1 cm1, 1149.0 cm1, 1242.5 cm1, and 1272.2 cm1 are due to the ester group in the polymer, which are the absorption peaks of stretching vibration. The bands at 2986.4 cm1, 1443 cm1, and 1386.4 cm1 are the characteristic absorption peaks of methyl. The characteristic absorption peak of methylene is at 2953.1 cm1. There is also a wide, strong absorption peak at 3452.4 cm1 because of a certain quantity of water in the polymer sample. In conclusion, the FT-IR spectrum of test sample is almost consistent with that of the standard PMMA. To clarify the polymerization mechanism of PMMA microspheres, as Table 1 and Fig. 3 shows, we also investigated the influences of the dosage of PVP, MMA, and initiator on the polymerization of PMMA. By comparing the morphologies of sample a, b, and c with different MMA dosage, it can be concluded that the MMA dosage influences the size scales of PMMA microspheres present after polymerization. The chain length of polymer in terms of primary nuclei can become longer as the monomer concentration becomes higher during the process of free radical polymerization, and as a result, the PMMA microspheres formed after polymerization have a large size scale and poor uniformity. Considering both of monodispersity and uniformity, the PMMA microspheres obtained by sample a were therefore used to assemble PMMA templates in Section 2.3. Similarly, Fig. 3a and d shows the influences of PVP dosage on the size and uniformity of PMMA microspheres. The weight ratio of PVP in sample a is higher than that in sample d, which means it is difficult to form the lager aggregates of these polymer
Fig. 4. Schematic of assemble PMMA array template.
nuclei in sample a due to the PVP-induced steric effect. The initiator also plays an important role on the morphology of the PMMA microspheres. Clearly, Fig. 3a and h shows this changes in morphologies. By comparing the morphologies of the sample a, e, f and g, it can be concluded that the reaction time and temperature are also important factors in polymerizing PMMA microspheres with uniform size. Fig. 3e–g shows the particle-evolution due to coagulation among the
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Intensity/a.u.
650
550
500 450 350 10 Fig. 5. FESEM images of PMMA microspheres template.
pre-existing microspheres. These microspheres were prepared under the similar conditions except for the reaction time. The PMMA microspheres in Fig. 3e were taken at 6 h, Fig. 3f, 7 h and Fig. 3g, 8 h, respectively. With longer reaction time, lager PMMA particles can be obtained proportionally. Numerous small particles are seen in Fig. 3e, whereas more aggregates and coagulated particles can be observed in Fig. 3g. By comparing Fig. 3a and g, it is clear that the diameter size and size distribution of PMMA microspheres are affected by the reaction temperature due to the degree of concentration of primary free radicals produced by the Fenton initiator. At higher temperatures (70 °C Fig. 3a), many free radicals were produced in a short time compared to lower temperatures (60 °C Fig. 3g), resulting in more primary nuclei, resulting in smaller PMMA particles with narrower size distribution. However, it should be noted that if too high temperature is reached, the polymerization is difficult to control, which resulted in the formation of coagulation. For the sake of the final uniform PMMA microspheres, we suggest that the reaction temperature has better to be below 80 °C. Fig. 4 shows the schematic of dip-drawing method used to assemble the template of polymer microspheres [13]. This
20
30
40
50
60
70
80
2 /° Fig. 7. XRD patterns of titania powders.
approach is based on the action of capillary force: put a clean substrate vertically into the emulsion of PMMA microspheres for several minutes and then draw it out with a slower drawing rate. Meanwhile, a meniscus region can be formed on the substrate due to the force of lateral capillary, which induces the PMMA microspheres to transfer from the bulk phase of emulsion to the surface of substrate and accumulate at the interface of three phases, that is, air, emulsion, and substrate. Fig. 5 shows the FESEM image of PMMA template prepared by this method. It is clear that there is a highly ordered array on the surface of substrate with respect to the PMMA template with a small number of spaces coexisting in it. Fig. 6 shows the influences of PMMA templates and sol concentration on the morphologies of macroporous TiO2 films. By comparing the morphologies of sample a and c, it can be concluded that the template of PMMA microspheres plays an important role on the morphology of the composite films. For sample a obtained without PMMA template, a mesoscale open framework composed
Fig. 6. FESEM images of macroporous TiO2 films with different sol concentrations (a) 2 mL TBOT without PMMA template, (b) 0.5 mL TBOT, (c) 2 mL TBOT, highmagnification, (d) 2 mL TBOT, low-magnification, (e) 4 mL TBOT, and (f) 8 mL TBOT.
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1.2
a
1.0
100
TG
90 80 70
0.8
60 50
0.6
40 0.4
TG/%
DTG/µg· min-1
DTG
30 20
0.2
10 0
0.0
-10 0
100
200
300
400
500
600
700
T/ 0C 1.2 1.0
100
b
TG
90
DTG
TG/%
DTG/µg·min
-1
0.8 0.6 0.4
80
0.2 70 0.0 0
100
200
300
400
500
600
700
T/ 0C 1.2
c
1.0
100
TG
90 80
0.8
70
0.6
60 50
0.4
TG/%
DTG/µg·min
-1
DTG
calcination. For sample f, it is difficult for TiO2 sol to be infiltrated into the interstices due to high viscosity, and the extra TiO2 sol can be accumulated on the surface of PMMA template to form the disordered framework structure ultimately after calcination. Although the viscosity of TiO2 sol in sample b is lowest with respect to the other samples, there is no enough dosage of TiO2 sol to form the TiO2 wall. XRD is always used to investigate the crystalline structure of nano TiO2 prepared at different calcination temperature. Fig. 7 shows the XRD patterns of the TiO2 powders as a function of calcination temperature. According to this graph, the crystalline phase of TiO2 is only in the anatase form at 450 °C, which shows a strong peak at 2h = 25.2° corresponded to the (1 0 1) diffraction peak of anatase phase. This figure also indicates that a small peak at 2h = 27.4° corresponded to the (1 1 0) diffraction peak of rutile phase with respect to the anatase to rutile phase transformation occurs at 550 °C. There is also another small peak at 2h = 31.0° corresponded to the (1 2 1) diffraction peak of brookite phase at the calcination temperature of 450–550 °C. This peak disappears at 650 °C, which means the transformation process of brookite phase. According to the previous report in combination with the photocatalytic activity of TiO2 [22], we selected 550 °C as the optimum calcination temperature in this work to remove the PMMA templates for preparing the macroporous films of TiO2. Fig. 8a–c shows the TG/DTG analysis for the samples of PMMA microspheres, TiO2 gel and TiO2/PMMA composite, respectively. Below 300 °C (Fig. 8a), about 19% mass loss of PMMA microspheres is due to volatile species (water and diethyl ether). Between 300 °C and 450 °C, a mass loss of about 75% arises from the decomposition of PMMA microspheres and other organic substance. Above 450 °C, the mass loss appears negligible. The mass loss of TiO2 gel below 220 °C is about 26% corresponding to the volatilization of water and hydrolization of tetrabutyl titanate (Fig. 8b). Between 220 °C and 550 °C, only 3% mass loss of TiO2 gel means the transformation process of anatase phase to rutile phase. Based on the TG/DTG analysis and XRD measure, the ordered macroporous TiO2 films were then calcined for completely removing templates as follows: 2.5 °C/min (20–220 °C), 220 °C for 1 h; 1 °C/min (220–450 °C), 450 °C for 1 h; 5 °C/min (450–550 °C), 550 °C for 4 h.
4. Conclusion
40 0.2
30 20
0.0
10 0
100
200
300
400
500
600
700
T/ 0C Fig. 8. TG/DTG profiles of PMMA microspheres (a), TiO2 gel (b), and TiO2/PMMA composite (c).
of worm-like nanoparticles and micropores can be observed, while sample c prepared with PMMA template shows the ordered framework with size of pores about 87 nm. From the FESEM images of sample b–f, it can be seen that the size of TiO2 wall becomes thicker as the dosage of Tetrabutyl titanate (TBOT) becomes higher, and the framework becomes more disordered. The reason for this phenomenon can be rationalized by analyzing the role of sol concentration-viscosity. The concentration of TiO2 sol in sample c is lower than that in sample e–f, which means the viscosity of TiO2 sol in sample c is lower than that in sample e–f. As a result, the sol is easier to be infiltrated into the interstices among the PMMA template by capillary force to form the ordered TiO2 wall after
We have presented a facile, reproducible synthetic method for producing macroporous TiO2 films with ordered framework by using the microspheres-assembled template of organic polymer. This method was based on the synthetic process of non-crosslinked, monodisperse PMMA microspheres via dispersion polymerization in the presence of Fenton reagent as an initiator. It was demonstrated that the PMMA microspheres obtained are perfect with uniform size about 87 nm and the size deviation is less than 5%. On the basis of experimental results, the mechanism of PMMA polymerization was therefore proposed, which is helpful to guide the preparation of polymer microspheres with desired uniform size. Comparing with the conventional emulsion polymerization, this novel synthesis technique exhibits some advantages such as simple and quick polymerization process without deoxygenation. Moreover, the templates with highly ordered arrays of PMMA microspheres assembled by capillary force with dip-drawing technique were applied to produce the macroporous TiO2 films with a variety of interesting morphologies by simply adjusting the dosage of TBOT. Both structures and morphologies of the composite films before and after calcination are investigated with XRD and FESEM, and the processes of mass loss of PMMA microspheres and TiO2 gel are also discussed with TG/DTG analysis. We also believe that the obtained ordered framework is promising to be used as the ultra-
H. Zhang et al. / Journal of Colloid and Interface Science 386 (2012) 73–79
filtration membranes showing active photocatalysis without intensive fouling. Further applications of the ordered macroporous TiO2 films in membrane devices for water treatment are in progress and will be addressed in a separate paper. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 20971113). References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Nature 452 (2008) 301. [2] A. Alem, H. Sarpoolaky, M. Keshmiri, J. Eur. Ceram. Soc. 29 (2009) 629. [3] S.H. Kim, S.Y. Kwak, B.H. Sohn, T.H. Park, J. Membrane Sci. 211 (2003) 157. [4] H.C. Flemming, Exp. Therm Fluid Sci. 14 (1997) 382. [5] J.-H. Li, Y.-Y. Xu, L.-P. Zhu, J.-H. Wang, C.-H. Du, J. Membrane Sci. 326 (2009) 659.
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[6] A. Mills, S. LeHunte, J. Photochem. Photobiol. A – Chem. 108 (1997) 1. [7] M. Paulose, L. Peng, K.C. Popat, O.K. Varghese, T.J. LaTempa, N. Bao, T.A. Desai, C.A. Grimes, J. Membrane Sci. 319 (2008) 199. [8] R. Vendamme, S.Y. Onoue, A. Nakao, T. Kunitake, Nat. Mater. 5 (2006) 494. [9] J.Q. Zhao, P. Wan, J. Xiang, T. Tong, L. Dong, Z.N. Gao, X.Y. Shen, H. Tong, Micropor. Mesopor. Mater. 138 (2011) 200. [10] O.D. Velev, A.M. Lenhoff, Curr. Opin. Colloid Interface Sci. 5 (2000) 56. [11] K. Sasahara, T. Hyodo, Y. Shimizu, M. Egashira, J. Eur. Ceram. Soc. 24 (2004) 1961. [12] Y. Cao, Y. Wang, Y. Zhu, H. Chen, Z. Li, J. Ding, Y. Chi, Superlattices Microstruct. 40 (2006) 155. [13] Z.F. Liu, Z.G. Jin, W. Li, J.J. Qiu, J. Zhao, X.X. Liu, Appl. Surf. Sci. 252 (2006) 5002. [14] X. Li, F. Tao, Y. Jiang, Z. Xu, J. Colloid Interface Sci. 308 (2007) 460. [15] A. Xiao, J. Yang, W. Zhang, J. Porous Mater. 17 (2010) 283. [16] W. Li, Z.G. Jin, Z.F. Liu, J.L. Yang, J.J. Qiu, J. Inorg. Mater. 21 (2006) 473. [17] D. Lan, Y.-R. Wang, Y. Yu, W.-J. Ma, C. Li, Chin. Phys. 16 (2007) 468. [18] Z. Wang, J. Guan, S. Wu, C. Xu, Y. Ma, J. Lei, Q. Kan, Mater. Lett. 64 (2010) 1325. [19] H. Kawaguchi, Prog. Polym. Sci. 25 (2000) 1171. [20] M. Fu, L. Deng, A. Zhao, Y. Wang, D. He, Opt. Mater. 32 (2010) 1210. [21] Y.W. Kang, K.Y. Hwang, Water Res. 34 (2000) 2786. [22] S. Karuppuchamy, J.M. Jeong, D.P. Amalnerkar, H. Minoura, Vacuum 80 (2006) 494.
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Fabrication of macroporous titanium dioxide film using PMMA microspheres as template Hongzhong Zhang a,⇑, Feng Dong a, Shaoming Fang a, Changming Ye a, Minghua Wang a, Haijun Cheng a, Zhouxiang Han a, Shengnan Zhai b a b
Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, People’s Republic of China University of Wollongong, Wollongong, New South Wales 2522, Australia
a r t i c l e
i n f o
Article history: Received 20 April 2012 Accepted 25 July 2012 Available online 3 August 2012 Keywords: PMMA microspheres Template Titanium dioxide Composite materials Thin films
a b s t r a c t A novel synthetic procedure is described for the fabrication of macroporous titanium dioxide (TiO2) films with an ordered, uniform pore framework comprised of nanocrystalline anatase mainly. The synthetic approach involved several processes. First, polymethyl methacrylate (PMMA) microspheres (87 nm) were synthesized by using a dispersion polymerization technique in the presence of Fenton reagent (FeSO4/ H2O2) as a novel initiator, which has advantages such as simple and fast polymerization process without deoxygenation. Next, the templates of PMMA microspheres were assembled on clean substrates by dipdrawing technique. Finally, the macroporous TiO2 films with the average size of pores about 87 nm were obtained by sol-dipping template method and calcination to remove the templates at 550 °C. The test results of X-ray diffraction indicate that the nanocrystalline of anatase formed after calcination. The mechanisms of PMMA polymerization and template formation were proposed. Furthermore, both structures and morphologies of the composite films were investigated with field emission scanning electron microscope, and the processes of the thermal decomposition of PMMA and TiO2 gel were also discussed with thermo gravimetric analysis. This ordered and uniform pore framework could be used as the promising ultrafilter membranes showing active photocatalysis without intensive fouling. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction In the coming decades, one of the most extensive problems afflicting people throughout the world is inadequate access to clean water and sanitation. However, water pollution has been becoming a more and more serious problem with the development of industries. Fortunately, recent activities in water treatment research offers hope in mitigating the impact of impaired waters around the world [1]. For example, membranes represent promising alternatives to conventional water and waste water treatment processes because membrane based separations are energy efficient and cost effective when optimized [2]. No matter which type of membranes, however, a major obstacle to further use in industrial operations is flux decline resulting from fouling [3]. Several types of fouling can occur in the membrane system, for example, crystalline fouling, organic fouling, particulate and colloidal fouling, and microbial fouling [4]. Several strategies to alleviate the membrane fouling have therefore been investigated. Various methods to reduce fouling have been carried out, which generally involve pretreatment of the feed solution, modification of the ⇑ Corresponding author. E-mail address: [email protected] (H. Zhang). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.07.080
membrane surface properties, optimization of module arrangement and process conditions, and periodic cleaning. Even after long periods of the developments, biofouling caused by microorganisms and biofilm accumulation on membrane still remains the main reason for flux decline [5]. Comparing with the conventional polymeric membranes, ceramic membranes are playing an important role in water treatment due to their many advantages, such as high temperature stability, high pressure resistance, good chemical stability, high mechanical resistance, and long life [2]. Alumina, titania, and zirconia are considered as the most common porous ceramic membrane materials. Among ceramic membranes, titanium dioxide (TiO2) has been the focus of intensive investigations in recent years, because of its photocatalytic effect that can decompose organic chemicals and destroy bacteria during the photocatalytic process illuminated semiconductor absorbs light and generates active species [6]. In general, the performance of membranes is attributed to two main factors: pore size distribution and porosity. For any one medium with specific size in water, more uniform in terms of membrane pores generate higher retention rate, higher density of pores, and more permeation efficiency. Therefore, the permeability and antifouling ability of TiO2 membranes with well-controlled and uniform pores can be significantly improved. Although porous TiO2
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membranes can be fabricated via nanoparticle sintering, as Paulose [7] pointed out, these membranes do not possess uniform size and shape of pores. Ultrathin nanoporous TiO2 membranes can be produced by preparing a self-supporting polymer–titania composite film on a nanoporous alumina substrate and then removing the polymer using chemical methods or plasma treatment [8]. However, these membranes are difficult to handle and the polymer removal disturbs the size and shape of pores. Crystalline mesoporous TiO2 membranes with well-defined size and shape of pores can be fabricated via surfactant templating, but are generally not strong enough for normal handling [7]. Over the past decades, polymer microspheres showing uniform size and large surface area have been often used as the templates to fabricate ordered macro-mesoporous materials/films [9–14]. In many cases, the performance of these polymer microspheres significantly depends on their size, shape, dispersity, and functional surface. For example, polystyrene (PS) microspheres have been mainly used as various templates to form ordered macroporous materials/films due to their good mono-dispersivity and controllability of size [13,15–18]. However, the materials or membranes with highly ordered meso-microporous structures using PS microspheres as templates have not been produced yet, because of their large size (beyond 500 nm generally) and poor functional surface [19]. Polymethylmethacrylate (PMMA) microspheres are therefore perfect alternative template associated with not only their functional surface, also easy removal of templates without any fracture of ordered porous frameworks [11]. However, until recently, only a limited number of results about templating ordered porous structures using PMMA microspheres have been reported. For example, Sasahara et al. [11] have used PMMA microspheres as a template to fabricate the macroporous ceramic thick films consisting of nanosized particles. In this case, the PMMA microspheres with a nominal uniform particle size of 800 nm were purchased commercially without providing any information of PMMA polymerization. In another case [20], although PMMA microspheres were prepared by in situ thermal polymerization, too long time (72 h) of synthesis and poor mono-dispersivity of microsphere size are the main defects in comparing with PS microspheres. In this work, we introduce a novel, reproducible synthetic method for preparing macroporous TiO2 films with ordered framework by using the microspheres-assembled template of organic polymer. Herein, non-crosslinked, monodisperse PMMA microspheres were fabricated via a dispersion polymerization method in the presence of Fenton reagent (FeSO4/H2O2) as an initiator. We demonstrate that the PMMA microspheres obtained are perfect with uniform size about 87 nm and the size deviation is less than 5%. Comparing with the conventional emulsion polymerization, this facile synthesis technique exhibits some advantages such as simple and quick polymerization process without deoxygenation due to using a novel initiator that has not been reported. Moreover, the TiO2 precursor solution prepared by sol–gel method was then infiltrated into the interstices among the PMMA templates by using sol-dipping method, followed by calcination to remove the template and produce the ordered macroporous TiO2 films. We also believe that the obtained ordered framework is promising to be used as the ultrafiltration membrane showing active photocatalysis without intensive fouling.
2. Materials and methods 2.1. Materials In the preparation of PMMA microspheres templates and TiO2 sol, the following materials were used: methyl methacrylate (MMA, CP, distillated before experiment) and polyvinylpyrrolidone
(PVP, K-30) were purchased from Sinopharm Chemical Reagent Co. Ltd. Ferrous sulfate (FeSO4, GR), hydrogen peroxide (H2O2, 30%), hydrochloric acid (HCl, AR), acetone (AR), glacial acetic acid (HAc, AR), diethyl ether (CH3CH2OCH2CH3, AR), and ammonia (NH3H2O, 25%) were obtained from Tianjin Kermel Chemical Reagent Co. Ltd. Tetrabutyl titanate (TBOT, CP) was purchased from Shanghai SSS Reagent Co. Ltd. Triethanolamine (TEA, AR) was purchased from Tianjin FuYu Fine Chemicals Co. Ltd. Absolute ethyl alcohol (EtOH, AR) was purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co. Ltd. 2.2. Polymerization of monodispersed PMMA microspheres Non-crosslinked, monodispersed PMMA microspheres were synthesized by using a dispersion polymerization technique. First, dispersion medium constituted by deionized water (30 mL) and diethyl ether (10 mL) with a certain proportion was heated at 70 °C in water bath, followed by 0.40 g PVP was added in the dispersion medium to be dissolved under magnetic stirring. Next, a certain amount of MMA purified for wiping off the inhibitor was added in the mixed solution, followed by the addition of HAc to adjust pH value at 3.0–4.0. Then, Fenton reagent used as an initiator, that is, 0.005 g FeSO4 and 100 lL H2O2, was also added in the mixed solution. Finally, the mixed solution was stirred for 8 h at 70 °C with stirring speed of 300 rpm. The resulting PMMA microspheres were remained suspended in the mother liquor until needed. 2.3. Assembly of the PMMA microspheres templates The PMMA microspheres templates were assembled on the substrates of monocrystalline silicon piece by the dip-drawing technique to observe the porosity and morphologies of these templates. Firstly, the substrates (1 3 cm) were cleaned ultrasonically for 30 min in the solution mixed with H2O2 (30%), NH3H2O, and deionized water with a certain proportion of 1:1:2 (volume ratio). Then, the clean and dried substrates were immersed vertically in the emulsion of PMMA for 5 min. Finally, the substrates were slowly drawn out (drawing speed of 0.1 cm/min) and dried at 30 °C for 15 min. After the procedure was performed as mentioned above, opalescent PMMA templates with prismatic colors like rainbow were obtained, depending on the angle of observation. 2.4. Preparation of TiO2 Sol TiO2 precursor solution was prepared by sol–gel method at room temperature according to the following procedure: TBOT was dissolved in the mixture solution of 20 mL EtOH and 3 mL TEA under vigorous stirring and kept for 2 h. Thus, the homogeneous, transparent, and stabilized TiO2 sol with yellowish green color was obtained. 2.5. Preparation of macroporous TiO2 films Sol-dipping template method was used to fill TiO2 sol into the interstices among the PMMA templates. First, the substrates of monocrystalline silicon with PMMA array template on it were immersed in TiO2 sol and kept for 1 min. Then, the substrates were drawn out from sol and dried at room temperature for 3 h to form the composite structure of thin film covered by PMMA microspheres. Finally, the resulting macroporous TiO2 films were obtained by calcination to remove the PMMA templates as the following recipe: calcination of the films was carried out in air with a heating rate of 2.5 °C/min starting from room temperature to 220 °C, maintained it at 220 °C for 1 h; 1 °C/min from 220 °C to 430 °C, maintained it at 430 °C for 1 h; 5 °C/min from 430 °C to
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550 °C and maintained it at 550 °C for 4 h, respectively. After calcination, the samples were cooled to room temperature in the furnace.
1272.2
1242.5
100
The IR spectrum of the PMMA synthesized by the recipe of Section 2.2 was recorded with a fourier transform infrared (FT-IR) spectrophotometer (Tensor27, Bruker). The diameter of PMMA microsphere was estimated using transmission electron microscopy (TEM, JSM-2001). Morphologies of the templates and macroporous films were examined using field emission scanning electron microscope (FESEM, JSM-7001F). X-ray diffraction (XRD) patterns of TiO2 powders were recorded on a Bruker D8 Advance X-ray diffractometer operated at 40 kV and 35 mA using Cu Ka radiation and nickel filter (k = 0.15406 nm). The thermal-decomposition behavior of the PMMA template filled with TiO2 precursor was measured using a thermogravimetric and differential thermal analysis (TG/DTA) instrument (Diamond, USA).
Transmittance/%
80
2.6. Characterization
988.5
60
843.3
2986.4 2953.1
40
1386.4
3452.4
1443.2 1193.1
20
1149.0 1731.6
0 4000
3500
3000
2500
2000
1500
1000
500
0
Wavenumber/cm-1 Fig. 2. FT-IR spectrum of PMMA sample.
Table 1 The recipes for polymerizing PMMA microspheres.
3. Results and discussion Fenton reagent is a redox initiation system consisting of Fe2+ and H2O2, which is extensively used in the oxidation treatment of waste water. The chain reaction between Fe2+ and H2O2 in acidic medium can catalyze OH free radical in which the oxidation potential is +2.8 V. The reaction process [21] is explained as follows:
Fe2þ þ H2 O2 ! Fe3þ þ OH þ OH
ð1Þ
Fe2þ þ OH ! Fe3þ þ OH
ð2Þ
H2 O2 þ OH ! H2 O þ OOH
ð3Þ
Fe2þ þ OOH ! Fe3þ þ OOH
ð4Þ
Fe3þ þ OOH ! Fe2þ þ Hþ þ O2
ð5Þ
The emulsion of homogeneous system in this work is composed of monomer (MMA), initiator (Fenton reagent), stabilizing agent (PVP), and dispersion medium (H2O and diethyl ether). When a given temperature is reached, the polymerization of AC@CA double bond in MMA could be carried out by OH caused by Fenton reagent. As the molecular weight of PMMA increases, the solubility of polymer decreases. Thus, when the chain length of PMMA is increased to a critical value, the polymer should be separated and precipitated from the dispersion medium. Then, the primary nuclear shells of
Sample MMA PVP no. (mL) (g)
H2O Diethyl Reaction Temperature FeSO4 H2O2 time (h) (°C) (mL) ether (g) (lL) (mL)
a b c d e f g h
30 30 30 30 30 30 30 30
4 6 8 4 4 4 4 4
0.40 0.40 0.40 0.20 0.40 0.40 0.40 0.40
10 10 10 10 10 10 10 10
8 8 8 8 6 7 8 8
70 70 70 70 70 70 60 70
0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.010
100 100 100 100 100 100 100 200
polymer are generated by PVP and held in the emulsion. Consequently, the volume of nuclear shell is increased with the ongoing polymerization because there are also MMA molecules and OH in the primary nuclear shell. Therefore, the microspheres of PMMA will be formed. Fig. 1a and b shows the images of transmission electron microscopy (TEM) and field emission scanning electron microscope (FESEM) of PMMA microspheres prepared in this work, respectively. It can be seen that the average diameter size of the PMMA microspheres obtained is about 87 nm, and the size deviation is less than 5%. The FT-IR spectrum of the polymer sample obtained is shown in Fig. 2. The band at 1731.6 cm1 is attributed to symmetric anhydride C@O stretching modes of the methacrylate group, and the
Fig. 1. TEM image (a) and FESEM image (b) of PMMA microspheres.
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Fig. 3. FESEM images of PMMA microspheres with different polymerization recipes.
bands at 1193.1 cm1, 1149.0 cm1, 1242.5 cm1, and 1272.2 cm1 are due to the ester group in the polymer, which are the absorption peaks of stretching vibration. The bands at 2986.4 cm1, 1443 cm1, and 1386.4 cm1 are the characteristic absorption peaks of methyl. The characteristic absorption peak of methylene is at 2953.1 cm1. There is also a wide, strong absorption peak at 3452.4 cm1 because of a certain quantity of water in the polymer sample. In conclusion, the FT-IR spectrum of test sample is almost consistent with that of the standard PMMA. To clarify the polymerization mechanism of PMMA microspheres, as Table 1 and Fig. 3 shows, we also investigated the influences of the dosage of PVP, MMA, and initiator on the polymerization of PMMA. By comparing the morphologies of sample a, b, and c with different MMA dosage, it can be concluded that the MMA dosage influences the size scales of PMMA microspheres present after polymerization. The chain length of polymer in terms of primary nuclei can become longer as the monomer concentration becomes higher during the process of free radical polymerization, and as a result, the PMMA microspheres formed after polymerization have a large size scale and poor uniformity. Considering both of monodispersity and uniformity, the PMMA microspheres obtained by sample a were therefore used to assemble PMMA templates in Section 2.3. Similarly, Fig. 3a and d shows the influences of PVP dosage on the size and uniformity of PMMA microspheres. The weight ratio of PVP in sample a is higher than that in sample d, which means it is difficult to form the lager aggregates of these polymer
Fig. 4. Schematic of assemble PMMA array template.
nuclei in sample a due to the PVP-induced steric effect. The initiator also plays an important role on the morphology of the PMMA microspheres. Clearly, Fig. 3a and h shows this changes in morphologies. By comparing the morphologies of the sample a, e, f and g, it can be concluded that the reaction time and temperature are also important factors in polymerizing PMMA microspheres with uniform size. Fig. 3e–g shows the particle-evolution due to coagulation among the
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Intensity/a.u.
650
550
500 450 350 10 Fig. 5. FESEM images of PMMA microspheres template.
pre-existing microspheres. These microspheres were prepared under the similar conditions except for the reaction time. The PMMA microspheres in Fig. 3e were taken at 6 h, Fig. 3f, 7 h and Fig. 3g, 8 h, respectively. With longer reaction time, lager PMMA particles can be obtained proportionally. Numerous small particles are seen in Fig. 3e, whereas more aggregates and coagulated particles can be observed in Fig. 3g. By comparing Fig. 3a and g, it is clear that the diameter size and size distribution of PMMA microspheres are affected by the reaction temperature due to the degree of concentration of primary free radicals produced by the Fenton initiator. At higher temperatures (70 °C Fig. 3a), many free radicals were produced in a short time compared to lower temperatures (60 °C Fig. 3g), resulting in more primary nuclei, resulting in smaller PMMA particles with narrower size distribution. However, it should be noted that if too high temperature is reached, the polymerization is difficult to control, which resulted in the formation of coagulation. For the sake of the final uniform PMMA microspheres, we suggest that the reaction temperature has better to be below 80 °C. Fig. 4 shows the schematic of dip-drawing method used to assemble the template of polymer microspheres [13]. This
20
30
40
50
60
70
80
2 /° Fig. 7. XRD patterns of titania powders.
approach is based on the action of capillary force: put a clean substrate vertically into the emulsion of PMMA microspheres for several minutes and then draw it out with a slower drawing rate. Meanwhile, a meniscus region can be formed on the substrate due to the force of lateral capillary, which induces the PMMA microspheres to transfer from the bulk phase of emulsion to the surface of substrate and accumulate at the interface of three phases, that is, air, emulsion, and substrate. Fig. 5 shows the FESEM image of PMMA template prepared by this method. It is clear that there is a highly ordered array on the surface of substrate with respect to the PMMA template with a small number of spaces coexisting in it. Fig. 6 shows the influences of PMMA templates and sol concentration on the morphologies of macroporous TiO2 films. By comparing the morphologies of sample a and c, it can be concluded that the template of PMMA microspheres plays an important role on the morphology of the composite films. For sample a obtained without PMMA template, a mesoscale open framework composed
Fig. 6. FESEM images of macroporous TiO2 films with different sol concentrations (a) 2 mL TBOT without PMMA template, (b) 0.5 mL TBOT, (c) 2 mL TBOT, highmagnification, (d) 2 mL TBOT, low-magnification, (e) 4 mL TBOT, and (f) 8 mL TBOT.
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1.2
a
1.0
100
TG
90 80 70
0.8
60 50
0.6
40 0.4
TG/%
DTG/µg· min-1
DTG
30 20
0.2
10 0
0.0
-10 0
100
200
300
400
500
600
700
T/ 0C 1.2 1.0
100
b
TG
90
DTG
TG/%
DTG/µg·min
-1
0.8 0.6 0.4
80
0.2 70 0.0 0
100
200
300
400
500
600
700
T/ 0C 1.2
c
1.0
100
TG
90 80
0.8
70
0.6
60 50
0.4
TG/%
DTG/µg·min
-1
DTG
calcination. For sample f, it is difficult for TiO2 sol to be infiltrated into the interstices due to high viscosity, and the extra TiO2 sol can be accumulated on the surface of PMMA template to form the disordered framework structure ultimately after calcination. Although the viscosity of TiO2 sol in sample b is lowest with respect to the other samples, there is no enough dosage of TiO2 sol to form the TiO2 wall. XRD is always used to investigate the crystalline structure of nano TiO2 prepared at different calcination temperature. Fig. 7 shows the XRD patterns of the TiO2 powders as a function of calcination temperature. According to this graph, the crystalline phase of TiO2 is only in the anatase form at 450 °C, which shows a strong peak at 2h = 25.2° corresponded to the (1 0 1) diffraction peak of anatase phase. This figure also indicates that a small peak at 2h = 27.4° corresponded to the (1 1 0) diffraction peak of rutile phase with respect to the anatase to rutile phase transformation occurs at 550 °C. There is also another small peak at 2h = 31.0° corresponded to the (1 2 1) diffraction peak of brookite phase at the calcination temperature of 450–550 °C. This peak disappears at 650 °C, which means the transformation process of brookite phase. According to the previous report in combination with the photocatalytic activity of TiO2 [22], we selected 550 °C as the optimum calcination temperature in this work to remove the PMMA templates for preparing the macroporous films of TiO2. Fig. 8a–c shows the TG/DTG analysis for the samples of PMMA microspheres, TiO2 gel and TiO2/PMMA composite, respectively. Below 300 °C (Fig. 8a), about 19% mass loss of PMMA microspheres is due to volatile species (water and diethyl ether). Between 300 °C and 450 °C, a mass loss of about 75% arises from the decomposition of PMMA microspheres and other organic substance. Above 450 °C, the mass loss appears negligible. The mass loss of TiO2 gel below 220 °C is about 26% corresponding to the volatilization of water and hydrolization of tetrabutyl titanate (Fig. 8b). Between 220 °C and 550 °C, only 3% mass loss of TiO2 gel means the transformation process of anatase phase to rutile phase. Based on the TG/DTG analysis and XRD measure, the ordered macroporous TiO2 films were then calcined for completely removing templates as follows: 2.5 °C/min (20–220 °C), 220 °C for 1 h; 1 °C/min (220–450 °C), 450 °C for 1 h; 5 °C/min (450–550 °C), 550 °C for 4 h.
4. Conclusion
40 0.2
30 20
0.0
10 0
100
200
300
400
500
600
700
T/ 0C Fig. 8. TG/DTG profiles of PMMA microspheres (a), TiO2 gel (b), and TiO2/PMMA composite (c).
of worm-like nanoparticles and micropores can be observed, while sample c prepared with PMMA template shows the ordered framework with size of pores about 87 nm. From the FESEM images of sample b–f, it can be seen that the size of TiO2 wall becomes thicker as the dosage of Tetrabutyl titanate (TBOT) becomes higher, and the framework becomes more disordered. The reason for this phenomenon can be rationalized by analyzing the role of sol concentration-viscosity. The concentration of TiO2 sol in sample c is lower than that in sample e–f, which means the viscosity of TiO2 sol in sample c is lower than that in sample e–f. As a result, the sol is easier to be infiltrated into the interstices among the PMMA template by capillary force to form the ordered TiO2 wall after
We have presented a facile, reproducible synthetic method for producing macroporous TiO2 films with ordered framework by using the microspheres-assembled template of organic polymer. This method was based on the synthetic process of non-crosslinked, monodisperse PMMA microspheres via dispersion polymerization in the presence of Fenton reagent as an initiator. It was demonstrated that the PMMA microspheres obtained are perfect with uniform size about 87 nm and the size deviation is less than 5%. On the basis of experimental results, the mechanism of PMMA polymerization was therefore proposed, which is helpful to guide the preparation of polymer microspheres with desired uniform size. Comparing with the conventional emulsion polymerization, this novel synthesis technique exhibits some advantages such as simple and quick polymerization process without deoxygenation. Moreover, the templates with highly ordered arrays of PMMA microspheres assembled by capillary force with dip-drawing technique were applied to produce the macroporous TiO2 films with a variety of interesting morphologies by simply adjusting the dosage of TBOT. Both structures and morphologies of the composite films before and after calcination are investigated with XRD and FESEM, and the processes of mass loss of PMMA microspheres and TiO2 gel are also discussed with TG/DTG analysis. We also believe that the obtained ordered framework is promising to be used as the ultra-
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filtration membranes showing active photocatalysis without intensive fouling. Further applications of the ordered macroporous TiO2 films in membrane devices for water treatment are in progress and will be addressed in a separate paper. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 20971113). References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Nature 452 (2008) 301. [2] A. Alem, H. Sarpoolaky, M. Keshmiri, J. Eur. Ceram. Soc. 29 (2009) 629. [3] S.H. Kim, S.Y. Kwak, B.H. Sohn, T.H. Park, J. Membrane Sci. 211 (2003) 157. [4] H.C. Flemming, Exp. Therm Fluid Sci. 14 (1997) 382. [5] J.-H. Li, Y.-Y. Xu, L.-P. Zhu, J.-H. Wang, C.-H. Du, J. Membrane Sci. 326 (2009) 659.
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