Surfactant-Free Synthesis of Macroporous TiO2 Films by a

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Surfactant-Free Synthesis of Macroporous TiO2 Films by a Photopolymerization-Induced Phase-Separation Method Jianxi Yao,*,†,‡ Fuzhi Wang,†,‡ Masahide Takahashi,§ and Toshinobu Yoko§ Renewable Energy School, North China Electric Power UniVersity, Beijing 102206, China, Beijing Key Laboratory of New Energy and Renewable Energy, Beijing 102206, China, and Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan ReceiVed: May 26, 2009; ReVised Manuscript ReceiVed: July 21, 2009

TiO2 films with controlled macroporous structures have been prepared from the precursor solution containing photomonomer dipentaerythritol pentaacrylate by a photopolymerization-induced phase-separation method (PIPS) in the absence of any surfactant and colloidal templates. The gel TiO2 film deposited from the precursor solution by dip-coating was irradiated with the ultraviolet light for some time. During the irradiation process, the polymerization of the photomonomer was induced, which resulted in the phase-separation in the film system. At the end of the polymerization reaction, two phases existed in the film, one was the emerging polymer rich phase, and another was the residual monomer-TiO2 oligomer-rich phase. After heat-treatment at 600 °C, the entire polymer decomposed and a well-defined interconnected macroporous TiO2 films could be obtained. X-ray diffraction, scanning electron microscopy, atomic force microscopy, thermogravimetric, and differential thermal analysis were used to characterize the macroporous TiO2 films. The results showed that the macroporous texture could be tuned by changing the composition of the precursor solution and the type of the reactive monomer. Polyvinylpyrrolidone was introduced into the system to slow the solvent evaporation. Highly macroporous TiO2 films prepared by the PIPS method exhibited much higher photocatalytic activity for the decomposition of methylene blue dye than the dense TiO2 film. 1. Introduction Unique electronic and optical properties of TiO2 film provided it with utility as photocatalysts,1–3 sensing materials,4 dielectrics,5 antireflection coatings,6 electrochromic films,7 “self-cleaning” coatings on windows or tiles,8 and anodes for solar cells.9–11 It is well known that film properties were highly dependent upon the preparation process and surface microstructure.12–14 Increasing the specific surface area of these devices is a promising way to improve the characteristics owing to the increased number of active sites. In the past decades, the synthesis of porous TiO2 films had experienced strong advances. Various chemical techniques had been employed, such as those based on hydrothermal crystallization,15 direct deposition,16,17 sputtering technology,18 ultrasonic spray pyrolysis,19 and sol-gel method.12,20–23 In the case of sol-gel method, the controlling porosity in the upper limit of the macroporous scale was achieved by the use of larger colloidal templates, such as latex or silica nanometric beads and emulsion, or foam templating.24,25 Additionally, incorporation of organic surfactant molecules into the precursor solution was an excellent way for obtaining porous films, since widely controlled microscopic structures might possibly be obtained by making the best use of the phaseseparated structure in the system. Mesoporous TiO2 film with 2D-hexagonal or cubic structure could be obtained by using ethanolic solutions of MCln (inorganic precursor) and nonionic amphiphilic block copolymers (template).26,27 Macroporous TiO2 films with modified structures were prepared by the sol-gel * To whom correspondence should be addressed. Tel: +86-10-51976816. Fax: +86-10-80795239. E-mail: [email protected]. † North China Electric Power University. ‡ Beijing Key Laboratory of New Energy and Renewable Energy. § Kyoto University.

dip-coating method from titanium alkoxide-based coating solution containing poly(ethylene glycol),20,28–30 polyoxyethylene,31 or nonylphenyl ether.32,33 From these earlier works, it has been proposed that the surfactant molecules played a crucial role in synthesizing porous TiO2 films. The surfactant agent forms a stable complex with Ti-oxo species. The phase-separation process is induced by the strong interactions between the inorganic oligomers and the organic polymer, which repel the solvent, and then, the macropores are generated by the evaporation of solvent. In order to extract a higher performance from porous TiO2 films with potential application, porosity control is urgently required in the film preparation process. However, despite the insight gained in the synthesis mechanism of titania macroporous films, some aspects still have to be considered. In the film systems, the controlled macropores with various sizes and distributions can be regarded as the result of the interaction between the polymerizable metalxane oligomer, solvent mixture, and incorporated surfactant molecules. Compared to the bulk systems, the overlap of the rapid solvent evaporation and polycondensation stages, deposition process, and large sensitivity to environmental conditions is the characteristics in the film systems.20,29,32–37 Until now, the control of the porosity of porous TiO2 films is still very difficult and faces a big challenge. There is a strong need for the development of innovative techniques that would allow the well-controlled synthesis of porous TiO2 films. In this paper, we have developed a method to prepare macroporous TiO2 films by a photopolymerization-induced phase-separation method (PIPS) in the absence of any surfactant and colloidal templates. Generally, the PIPS method was widely used for the fabrication of the microstructures in the polymerdispersed liquid crystal films.38–40 In the PIPS process, the phase-

10.1021/jp904887v CCC: $40.75  2009 American Chemical Society Published on Web 08/11/2009

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Figure 1. Chemical structures of DPEPA.

separation takes place at constant temperature and is driven by the polymerization instead of the usual thermal quench. In our studies, the small molecular organic monomer and the polymerization initiator were mixed with the titanium alkoxide ethanol solution. After dip-coating, the as-prepared TiO2 gel film was irradiated by the ultraviolet light. After that, the polymerization of the organic monomer was induced by the uniformly irradiation. At some point in time after the emerging polymer started to grow, the miscibility gap was breached and the phaseseparation of discrete phases occurred. Such a reacting system should be treated as a two-phase system as it contained the residual monomer-TiO2 oligomer rich phase and the emerging polymer-rich phase. After heat treatment, the removal of the emerging polymer could lead to the formation of macroporous TiO2 films. The morphology of the as-prepared TiO2 films could be easily controlled by the adjustment of the polymerization condition, the initial composition of component, and the type of monomers. The as-prepared macroporous TiO2 film demonstrated high photocatalytic activity toward degrading methylene blue. 2. Experimental Section 2.1. Preparations of Porous TiO2 Films. Titanium tetraisopropoxide (TIP, Ti(OiC3H7)4) (Wako) was used as the titanium source. Dipentaerythritol pentaacrylate (DPEPA) (Aldrich) was used as the photomonomer, whose structural formula is shown in Figure 1. 2,2′-Azobisisobatyronitrile (AIBN) (Wako) was used as the radical initiator. All of the reagents were used without further purification. The mixtures of water, ethanol, TIP, nitric acid, dimethylformamide (DMF) (Wako), DPEPA, AIBN, and polyvinylpyrrolidone (PVP) k30 (Wako), whose compositions were listed in Tables 1 and 2, were used as the precursor solution. In order to obtain the new emerging polymer with similar molecular weight, the ratio of DPEPA to AIBN was maintained to be the same in the solution. First, TIP was dissolved in DMF and half of the prescribed amount of ethanol. After that, a mixture of the other half of ethanol and water and 60 wt % aqueous of nitric acid was added dropwise to the former solution under the ice-cooled conditions with vigorous stirring. Then, AIBN was dissolved in the solution. A prescribed amount of PVP and DPEPA was added to the solution. The container was sealed and placed at room temperature for 30 min under stirring to completely dissolve PVP and DPEPA. Before dip-coating, the sol was aged for 30 min. Then the solutions were placed in a dip-coating chamber. Silica glasses (40 mm × 40 mm × 0.1 mm) with a heat resistance temperature of higher than 800 °C were used as the substrates. A withdrawal speed of 6 cm/min was employed. After being dip-coated, the resultant gel films were irradiated by UV light for 30 min. In the present study, a high-pressure mercury lamp (200 W, Philips HPK, λ ) 350-450 nm, the light intensity on the films is 30 mW/cm2) was used as the ultraviolet light source. After irradiation, the films were directly subjected into an electric oven preheated at 600 °C for 10 min.

Yao et al. The steps of the experimental process are described schematically in Scheme 1. In order to understand the solvent evaporation rate of the gel films, the gel films were moved to a balance after the dip-coating operation as soon as possible. After that, the weight loss (g/ cm2) of the films was recorded every 5 s. 2.2. Characterization of Porous TiO2 Films. The crystal structures of the as-prepared films were investigated using an X-ray diffractometer (XRD, RINT 2100-S, Rigaku, Japan) with Cu KR radiation operated at 40 kV and 40 mA. The morphology was directly observed using a scanning electron microscope (SEM, JSM-6301F, Hitachi, Japan) with an accelerating voltage of 15 kV and atomic force microscopy (AFM, JSPM-5300, Witec, Germany) in the contact mode. AFM image was performed using pyramidally shaped silicon nitride tips (4 µm base, 4 µm height, aspect ratio ∼1:1, radius < 50 nm) on silicon nitride cantilevers. The thermogravimetric and differential thermal analysis (TGDTA) was performed using a Thermo plus TG 8210 (Rigaku, Japan). Measurement was made at a heating rate of 10 °C/min from 30 to 800 °C under air flow. The gel for the TG-DTA measurements was scratched off from the irradiated gel films by a knife. The thickness of the films was measured using a profilometer (SE-30D, Kosaka Lab, Japan). 2.3. Photocatalytic Experiment. To examine the photocatalytic activity of the obtained macroporous TiO2 films, a synthetic quartz cell of 3 cm3 was used as the reaction container. The cell was filled with 2 mL of methylene blue (MB) ([(CH3)2N]2C12H6NS(Cl)) solution with the pH value of 5.3 (6 × 10-5 M), where the film sample of 6 × 15 mm in size was immersed. The photodegradation of MB as a function of time under the irradiation of Xe lamp of 150 mW/cm2 was monitored by UV-visible spectrophotometer (Hitachi UV-3500). The MB concentration was determined by measuring the maximum absorbance around λ ) 664 nm in UV-vis absorption spectra. 3. Results and Discussion 3.1. Morphology of TiO2 Films. SEM images of TiO2 films prepared from the solution without DPEPA and S4 solutions (shown in Table 1) under different reaction conditions are presented in Figure 2. From Figure 2a, it could be seen that only a dense film could be obtained when no monomer was added into the system. Figure 2b indicated that no pores but many big cracks existed on the surface of TiO2 film prepared from S4 solution containing DPEPA but without UV irradiation. The SEM image of the film prepared from S4 solution with UV irradiation but without heattreatment (Figure 2c) showed that the surface of the gel film was not flat but rugged, and two domains could be observed (dark part and bright part). However, after being heat-treated at the temperature 600 °C, it was clear that many interconnected macropores appeared on the surface of the film with few cracks (Figure 2d).The mean pore size was about 1.0 µm. At a dipcoating speed of 6 cm/min, the average film thickness of the four films was measured to be about 1.1 µm. Before irradiation, the TiO2 oligomer, the DPEPA monomer, and the solvent were completely miscible with each other and the system might be treated as a single phase. In this case, a dense film with many cracks could be obtained (Figure 2b). The cracks were considered to be created when the gel film was too soft to stand for the horizontal tensile stress emerged during the heat treatment.20 DPEPA is a kind of photomonomer, which is very easily polymerized by the ultraviolet irradiation. The polymerization of DPEPA in the film containing AIBN

Surfactant-Free Synthesis of Macroporous TiO2 Films

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TABLE 1: Composition of the Coating Solutions by Changing the Amount of DPEPA solution

Ti(OC3Hi7)4 (molar ratio)

ethanol (molar ratio)

H2O (molar ratio)

HNO3 (molar ratio)

DMF (molar ratio)

AIBN (g)

PVP (%)

DPEPA (molar)

S1 S2 S3 S4

1 1 1 1

8 8 8 8

3 3 3 3

0.5 0.5 0.5 0.5

4 4 4 4

0.013 0.026 0.039 0.052

5 5 5 5

0.0025 0.005 0.0075 0.01

TABLE 2: Composition of Coating Solutions by Changing the Molar Ratio of HNO3 solution

Ti(OC3Hi7)4 (molar ratio)

ethanol (molar ratio)

H2O (molar ratio)

HNO3 (molar ratio)

DMF (molar ratio)

AIBN (g)

PVP (%)

DPEPA (molar)

H5 H4 H3 H2

1 1 1 1

8 8 8 8

3 3 3 3

0.5 0.4 0.3 0.2

4 4 4 4

0.052 0.052 0.052 0.052

5 5 5 5

0.01 0.01 0.01 0.01

SCHEME 1: Preparation Process of Macroporous TiO2 Film by the PIPS Method

could be considered to be due to the photolysis of AIBN, which produced a radical interaction with the DPEPA monomer.41,42 The emergence of the macropores in the films prepared by UV irradiation might be treated as a result of a polymerization process of DPEPA coupled with the hydrolysis and polycondensation of TIP. The hydrolysis and polycondensation process of TIP were strongly determined by the solution compositions and not affected by the UV light. During the PIPS reaction

Figure 2. SEM images of the TiO2 films prepared from (a) solution without DPEPA, with UV irradiation and heat-treatment, (b) S4 solution, without UV irradiation and with heat-treatment, (c) S4 solution, with UV irradiation but without heat-treatment, (d) S4 solution, with UV irradiation and heat-treatment.

process, the degree of DPEPA polymerization progressively increased with increasing irradiation time. As the photoinduced polymerization reaction proceeded, the increasing molecular weight of poly-DPEPA caused the system to be immiscible and drove the phase separation between the residual monomer-TiO2 oligomer and the emerging poly-DPEPA. Eventually, the emerging poly-DPEPA would segregate from the residual monomer, as well as from the TiO2 oligomer component, to form two kinds of phase consisting of the existing poly-DPEPA rich phase and the residual monomer-TiO2 oligomer rich phase. This phenomenon was very similar to the observation in polymer dispersed liquid crystal films system.41,43,44 The rugged morphology in Figure 2c might be caused by the different contrasts between the two phases. After being heat-treated at 600 °C for the thermal decomposition of polymers, a well-defined interconnected macroporous TiO2 film could be obtained. The small number of the cracks was observed on the surface of the irradiated film since the gel film might be reinforced by the polymerization of the monomer. It should be pointed out that, in previous investigations, in order to produce macroporous films in a reproducible way, it was necessary to accurately control three most important synthesis parameters, apart from the solution composition: (a) withdrawal speed, (b) solution temperature, and (c) %RH inside the dip-coating chamber.23,32–37 In the present research, such unique macroporous structures were quite reproducible at certain compositions irrespective of the withdrawal speed, the solution temperature, and the humidity varying in a relatively wide range. The structure of the film was strongly affected by the solution composition. Figure 3 shows the AFM image (10 µm × 10 µm) of the macroporous TiO2 film shown in Figure 2d. It was obvious that the pores with the mean pore size about 1.0 µm were threedimensionally penetrating, which agreed well with the SEM

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Figure 3. AFM image of the porous TiO2 films prepared from S4 solution with UV irradiation and heat-treatment.

Figure 5. SEM images of the porous films prepared from the solution with different amounts of DPEPA of (a) 0.0025, (b) 0.005, (c) 0.0075, and (d) 0.01 mol and 0.5 M of HNO3.

Figure 4. TGA-DTA curves of solid xerogel obtained from S4 solution in an air atmosphere.

result. The average depth of the pores was estimated to be ∼950 nm. Such a three-dimensionally extended macroporous structure might be an option to increase the surface area.20 3.2. TG-DTA investigation. To examine the effect of the heat-treatment temperature on the properties of the macroporous TiO2 films, TG-DTA were performed on the solid xerogel scratched off from the as-deposited TiO2 films shown in Figure 2c. The TG-DTA results were shown in Figure 4. In the temperature ranging from 30 to 300 °C, about 25% weight loss was observed, accompanying several endothermic peaks. This was probably due to the evaporation of the residual solvents, physically adsorbed water and liberation of pending alkoxy groups. Three obvious exothermic peaks occurred between 300 and 550 °C, accompanying about 45% weight loss. These exothermic peaks were attributed to the decomposition of the residual monomer, PVP, and the photopolymerized species. The exothermic peak at ∼480 °C was due to the amorphousanatase transition of TiO2. From Figure 4, for the phase transition from anatase to rutile, no peak was observed. The TG-DTA results clearly showed that most of the organic phase was removed from the film upon calcination at 600 °C. 3.3. Effect of DPEPA. The formation of the porous structure in TiO2 films was strongly depended on the amount of DPEPA. It was observed that no pores appeared in the TiO2 film prepared from the solution without DPEPA (Figure 2a). Figure 5 shows the SEM images for the TiO2 films prepared from the precursor solutions containing different additions of DPEPA. From Figure 5, it could be seen that different porous structures developed in the films after DPEPA was introduced. The SEM image of the TiO2 film prepared from S1 solution (0.0025 mol DPEPA) in Table 1 (Figure 5a) indicated that only a few isolated macropores with a mean size of about 1.1 µm appeared on the surface of the film. Further SEM observations were carried out on the

macroporous TiO2 films prepared from the precursor solution containing 0.005 (S2 solution), 0.0075 (S3 solution), and 0.01 mol (S4 solution) of DPEPA, which are shown in panels b, c, and d of Figure 5, respectively. From these images, it was clear that the density of the pores increased with increasing the amount of DPEPA; on the other hand, the pore size changed a little. In Figure 5d, a typical interconnected macroporous structure with the pore size about 1.0 µm could be clearly observed. The change in the morphologies of the films shown in Figure 5 from few isolated macropores to abundant interconnected macropores clearly reflected the monomer concentration influence in the system. Unambiguous experimental evidence proved that the intramolecular cross-linking density during the photoinduced polymerization process would lead to the structure transformation.44–46 In the present system, increasing the amount of DPEPA resulted in an increased cross-linking density, which was beneficial to the connection of the poly-DPEPA domain. At the same time, the high spatial extension of monomer and poly-DPEPA resulted from high cross-linking also played an important role on the connection of the domain.45,46 After heattreatment, many interconnected macropores were produced in the film because the connected poly-DPEPA domains were decomposed completely at 550 °C. However, the high density of macroporous structure was decayed at lower DPEPA concentration, which was due to the low cross-linking density and low molecule spatially extension. By XRD analysis of the films obtained from the coating solutions with different amounts of DPEPA, it was confirmed that the increment in the amount of DPEPA influenced the crystallization of the TiO2 films. Figure 6 showed the XRD patterns of the as-prepared TiO2 films prepared by solution S1, S2, S3, and S4. In Figure 6a, there were five peaks at 2θ of 25.3°, 38.6°, 48.1°, 54.5°, and 55.3°, which were attributed to the anatase phase (JCPDS, no. 211272).6 When the amount of DPEPA was increased to 0.005 (Figure 6b), 0.0075 (Figure 6c), and 0.01 mol (Figure 6d), a small peak at 27.4° arising from the rutile (110) peak was observed (JCPDS, no. 21-1276).20 The intensity of this peak increased with an increase in the amount of DPEPA. This could be explained by the exothermic reactions occurring during the organic component removal, that is, the combustion of the

Surfactant-Free Synthesis of Macroporous TiO2 Films

Figure 6. XRD patterns of the porous films prepared from with different amount of DPEPA of (a) 0.0025, (b) 0.005, (c) 0.0075, and (d) 0.01 mol and 0.5 M of HNO3.

Figure 7. SEM images of the porous films prepared from solution with molar ratio of HNO3 of (a) 0.5, (b) 0.4, (c) 0.3, and (d) 0.2 M and 0.01 mol of DPEPA.

organic component emitted heat locally, causing an increase in the temperature of the films. The phase transformation from anatase to rutile was considered to be enhanced by such an additional heat. 3.4. Effect of the Molar Ratios of HNO3. The SEM images of the films prepared from precursor solutions with different concentrations of HNO3 (Table 2, solution H5, H4, H3, and H2) were shown in Figure 7. The morphologies of the prepared films varied from interconnected macropores to dense film with the decrease in the molar ratio of HNO3 from 0.5, 0.4, and 0.3 to 0.2. Simultaneously, the mean pore size decreased from 1.0 and 0.8 µm to 0.6 µm with decreasing the molar ratio of HNO3. The molar ratios of HNO3 could strongly affect the solidification of the film and the average molecular weight of the emerging poly-DPEPA. First, in principle, the macroscopic domains were developed if the solidification of the inorganic polymeric phase was produced after the phase separation (in the present study, it referred to the polymerization rate), which originated the pores. If the solidification was produced before the phase separation, films with no apparent macropores were obtained.20,23 Film fluidity was required during the photopolymerization process to help the mobility of the organic monomer and obtain a deeper polymerization degree. In higher molar ratios of HNO3, the solidification rate of the film was relative low, which was favorable for the mobility of monomer,

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Figure 8. Time dependence of the weight loss (g/cm2) of the TiO2 films prepared from the solution (a) containing 5 wt % PVP and (b) no PVP.

and then large numbers of the larger size poly-DPEPA domains could be obtained. With the decrease of HNO3 molar ratio, the increasing solidification rate resulted in the smaller and fewer poly-DPEPA domains. When the HNO3 molar ratio was 0.2, the solidification rate was much faster than that of the phase separation. As observed in Figure 7d, only dense film could be obtained because a tightly cross-linked gel network developed before the polymer domains formed. Second, increasing HNO3 concentration could enhance the average molecular weight of the poly-DPEPA, which is favorable for the formation of larger polymer domain. After heat-treatment, the polymer was removed and the macropores with larger mean pore size could be obtained. Decreasing the concentration of HNO3 could suppress the photopolymerization reaction, which would lead to the smaller polymer domain and then smaller pore size. 3.5. Effect of PVP. In order that the present film subjected to irradiation underwent phase separation, a relatively long time must be required due to the low diffusion and mobility of the oligomer and monomer. Therefore, retardation of the film solidification and the solvent evaporation became very important for the formation of the phase-separation morphology in the film. The solvent evaporation in a typical sol-gel coating deposition was generally completed within several tens of seconds.47 In the present study, we introduced PVP into the precursor solution to reduce the solvent evaporation rate in order to obtain a wet, liquid-like coating lasting for several hours through strong hydrogen bonding of the compound with water and ethanol. The function of PVP had been proved by some previous studies.48,49 We have compared the evaporation rates of the solvent of the film deposited from the precursor sol containing 5 wt % PVP and without PVP. Figure 8 showed the time dependence of the weight loss (g/cm2) of the TiO2 films deposited from solutions containing PVP (Figure 8a) and without (Figure 8b) PVP. The weight loss was caused by the solvent evaporation in the film. The measurement was operated at room temperature and the time origin was taken as the moment of coating. The evaporation of solvent and water was completed with 180 s in the film without PVP (Figure 8b), but this time was significantly increased with the addition of PVP (Figure 8a) even after 1600 s. After 1600 s, only around 50% of the solvent evaporated. The films deposited with the above method remained wet for several hours after deposition, which kept the monomer mobility and afforded sufficient time for the phase separation. Before heattreatment, the film was dried and completely solidified. In the low PVP addition (4.5 wt %), no uniform interconnected macropores were observed (Figure S1, Supporting Information), which was due to the higher solvent evaporation rate. When

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Figure 9. (a) Chemical structures of PETA, TPGDA, and HDDA. SEM image of the films prepared by (b) PETA, (c) TPGDA, and (d) HDDA.

the content of PVP was increased to 5.5 and 6.5 wt %, parts of the film peeled off from the substrate, which was due to the higher thickness of the films. It was very interesting that the peeled parts of the film showed a kind of honeycomb-like structure (Figures S2 and S3, Supporting Information). With the increase of the amount of PVP, from the XRD result, it could be seen that the intensity of the rutile peak at 27.4° increased (Figure S4, Supporting Information), which was due to the combustion of the PVP component emitting heat locally, causing an increase in the temperature of the films. The phase transformation from anatase to rutile was considered to be enhanced by such an additional heat. 3.6. Effect of the Types of the Monomer. In order to investigate the diversity of film morphologies induced by variation of the monomer, pentaerythritol tetraacrylate (PETA), tripropyleneglycol diacrylate (TPGDA), and 1,6-hexanediol diacrylate (HDDA) with different functionality were used to prepared the porous TiO2 films. Generally, the greater the amount of double bonds means the higher of the monomer functionality. Compared to TPGDA and HDDA, DPEPA and PETA have higher functionality, cross-linking density, and molecular weight. The structural formulas of the monomer are shown in Figure 9a. The SEM images of the as-prepared films are shown in Figure 9b (PETA), c (TPGDA), and d (HDDA), whose precursor solution composition were the same as the S4 solution shown in Table 1. The SEM image of the TiO2 film prepared using PETA as reactive monomer (Figure 9b) indicated that interconnected macropores with the mean pore size about 0.9 µm could be obtained. When TPGDA was used as the reactive monomer, a film with many cracks could be prepared

and the mean pore size was around 0.4 µm. The SEM image of the film prepared by HDDA revealed that only few isolated macropores with a mean size about 1.0 µm appeared on the surface of the film. The interconnected structure was strongly decayed in this sample. The transformation in the morphologies of the films shown in Figure 9 clearly reflected the monomer effect on the system. Many previous studies have proved that the functionality of the monomer had great effect on the polymerization process of the monomer. Under the same reaction conditions, the monomer with higher functionality was easy to polymerize.41,49 In the present study, DPEPA and PETA have higher functionality and intramolecular cross-linking density, which was helpful for the polymerization of monomer and connection of new emergence polymer. In this case, TiO2 films with well-defined interconnected macropores could be fabricated. However, the interconnected macroporous structures were highly destroyed when monomers with lower functionality were used due to the low polymerization degree and low cross-linking density. 3.7. Photocatalytic Activities of the Porous TiO2 Films. The semiconductor TiO2 has been proven to be an excellent photocatalytic material, which degrades many organic compounds and in some cases decomposes completely under UV irradiation.31 For this purpose, MB is often used as the probe of photocatalytic activity.12 The absorption spectra of MB during the photodegradation process by TiO2 film prepared by S4 solution (Figure 2d) under UV irradiation was shown in Figure 10a and the time variation of the normalized peak height were shown in Figure 10b curve VI. In order to compare the photocatalytic activities of the porous TiO2 films prepared by

Surfactant-Free Synthesis of Macroporous TiO2 Films

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15627 the withdrawal speed, the solution temperature, and the humidity. Changing of the concentration of DPEPA monomer and the molar ratios of HNO3 in the precursor solution made it possible to control the subsequent structure of the macroporous TiO2 films. The controllability of the film structure could also be realized by increasing the number of the poly-DPEPA domain and adjusting the solidification rate of the film. PVP was introduced to the precursor solution to reduce the solvent evaporation, which kept the monomer mobility and afforded sufficient time for the phase separation. Monomers with higher functionality were favorable for the formation of the films with highly density macropores. The as-prepared porous TiO2 films were more highly efficient to decompose MB dyes than the dense film due to the high porosity. This PIPS method modified by varying the type and the concentration of the monomer template is simple, reliable, and useful in the preparation of porous TiO2 thin films with high photocatalytic activity. Acknowledgment. This work was sponsored by the National Natural Science Foundation of China (20876040, 20606035), the Scientific Research Foundation for the Returned Overseas Ministry of Education of China, General Program of “211 Project” of North China Electric Power University, and Japan Society for the Promotion of Science.

Figure 10. (a) Time-resolved absorption spectra for the MB bleaching process with the TiO2 film prepared from 0.01 mol DPEPA and 0.5 M HNO3 solutions. (b) Photodegradation behavior of MB (I) without TiO2 film and using (II) conventional dense TiO2 film and (III) TiO2 film prepared by HDDA, (IV) TiO2 film prepared by TPGDA, (V) TiO2 film prepared by PETA, and (VI) TiO2 film prepared by DPEPA.

TABLE 3: Rate Constants of Photocatalytic Reaction by Using Different Films sample used for the photocatalytic reaction

rate constants of photocatalytic reaction (×10-3 min-1)

no film dense film film prepared by HDDA film prepared by TPGDA film prepared by PETA film prepared by DPEPA

1.28 1.99 3.05 3.51 4.04 7.82

different monomer, the photocatalytic results of these samples were also shown in Figure 10b. As shown in Figure 10b curve I, only weak photolysis of MB happened in the absence of TiO2 film. From Figure 10b curves II-VI, it is clearly indicated that the activity of the macroporous TiO2 thin film was much higher than that of the conventional dense TiO2 thin film. The rate constants of photocatalytic reaction by using different films were shown in Table 3. It could be concluded that the photocatalytic properties were improved with the increase of monomer functionality used. This fact could be interpreted in terms of possessing much higher specific surface area available for photocatalytic activity in the macroporous film prepared by DPEPA, which could be recognized from the SEM observation (Figure 2d). 4. Conclusions Highly interconnected macroporous TiO2 films have been prepared by a PIPS method from the solution containing photomonomer DPEPA. Such unique macroporous structures were quite reproducible at certain compositions irrespective of

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