A Vapor Phase Hydrothermal Modification Method Converting a

Jun 4, 2009 - †Griffith School of Environment and Australian Rivers Institute, Gold Coast Campus, Griffith University,. QLD 4222, Australia, and ‡...
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A Vapor Phase Hydrothermal Modification Method Converting a Honeycomb Structured Hybrid Film into Photoactive TiO2 Film Huijun Zhao,*,† Yanming Shen,†,‡ Shanqing Zhang† and Haimin Zhang† †

Griffith School of Environment and Australian Rivers Institute, Gold Coast Campus, Griffith University, QLD 4222, Australia, and ‡Department of Chemical Engineering, Shenyang Institute of Chemical Technology, 11th Street, Shenyang Economic Development Zone Shenyang 110142, P.R. China Received April 16, 2009. Revised Manuscript Received May 13, 2009 Transforming an organic/inorganic hybrid material into a pure inorganic material without losing its original structure is of interest for a range of applications. In this work, a simple and effective vapor phase hydrothermal method was developed to transform a 3D honeycomb structured PS/TTIP hybrid film into a photoactive TiO2 film without dismantling the originally templated 3D structure. The method utilizes the vapor phase hydrothermal process to create titania network/clusters with sufficient mechanical strength via the formation of Ti-oxo bridges. The organic components of the sample can be removed by means of pyrolysis while perfectly maintaining the original 3D honeycomb structure. The resultant film can be directly used for photocatalysis applications and could be further modified for other applications. In principle, this method can be used to preserve 3D structures of other organic/ inorganic hybrid films during their conversion to pure inorganic films via a pyrolysis process, if mechanically strong networks can be formed as a result of hydrolysis reactions. The ability to preserve the preferred 3D structure during the subsequent conversion processes enables realization of the full benefit of unique architectures created by a templating method.

Introduction Synthesis of highly ordered, long-ranged periodic porous TiO2 films has been a topic of interest to researchers in multidisciplinary fields due to the high application potentials of such materials to catalysis, batteries, supercapacitors, chemical sensors, membrane, and photonic band-gap materials. Numerous templating synthesis methods have been proposed to obtain highly ordered porous TiO2 films by employing a variety of polymeric surfactants as the structure-directing agents.1-9 The composition of the resultant films obtained via such types of templating methods is organic/inorganic hybrid. These hybrid films have been suggested for direct applications where multifunctional materials are required.1,10-13 Nevertheless, they are often *Corresponding author: Fax 61 7 5552 8067, Tel 61 7 5552 8261, e-mail [email protected]. (1) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. J. Mater. Chem. 2005, 15, 3559. (2) Crepaldi, E. L.; de Soler-Illia, G. J.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770. (3) Yuwono, A. H.; Zhang, Y.; Wang, J.; Zhang, X. H.; Fan, H.; Ji, W. Chem. Mater. 2006, 18, 5876. (4) Imhof, A.; Pine, D. J. Nature (London) 1997, 389, 948. (5) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature (London) 1998, 396, 152. (6) Holland, B. T.; Blanford, C.; Stein, A. Science 1998, 281, 538. (7) Li, X.; Lau, K. H. A.; Kim, D. H.; Knoll, W. Langmuir 2005, 21, 5212. (8) Cheng, Y.-J.; Gutmann, J. S. J. Am. Chem. Soc. 2006, 128, 4658. (9) Choi, S. Y.; Lee, B.; Carew, D. B.; Mamak, M.; Peiris, F. C.; Speakman, S.; Chopra, N.; Ozin, G. A. Adv. Funct. Mater. 2006, 16, 1731. (10) Yano, S.; Iwata, K.; Kurita, K. Mater. Sci. Eng., C 1998, C6, 75. (11) Innocenzi, P.; Lebeau, B. J. Mater. Chem. 2005, 15, 3821. (12) Angelome, P. C.; Aldabe-Bilmes, S.; Calvo, M. E.; Crepaldi, E. L.; Grosso, D.; Sanchez, C.; Soler-Illia, G. J. A. A. New J. Chem. 2005, 29, 59. (13) Zhang, Y.; Wang, C. Adv. Mater. 2007, 19, 913. (14) Zukalova, M.; Zukal, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Graetzel, M. Nano Lett. 2005, 5, 1789. (15) Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A. Adv. Funct. Mater. 2004, 14, 335.

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employed as the precursor/template to obtain pure inorganic films.2,14-16 Under such circumstances, the organic component is removed in a subsequent fabrication process, commonly by means of solvent extraction or pyrolysis.2,3,8,15-17 However, the effectiveness of the template may be diminished because the original structure could be altered or even destroyed during the removal process, especially when the removal of organic component is carried out via a pyrolysis process.2,3,8,15-19 It is therefore highly desirable to develop a fabrication method that is capable of preserving the originally created templating structure during the organic component removal process to capture the full benefit of templating methods. In this work, we present a vapor phase hydrothermal (VPH) method that can be used to transform a three-dimensional (3D) structure of an organic/inorganic hybrid film into a highly photoactive pure TiO2 film while preserving the original structure created by the structure-directing agent. Figure 1 schematically illustrates the preparation procedures and the transformation route of a 3D honeycomb structure hybrid film of monocarboxyterminated polystyrene (PS) and titanium tetraisopropoxide (TTIP) into a pure TiO2 film. The breath figures (BFs) templating method was employed to create the large 3D honeycomb structure. This large 3D structure was purposely selected to demonstrate the effectiveness of the proposed VPH method as the preservation of a large 3D macrostructure is more difficult than that of small dimensional structures. (16) Shibata, H.; Ogura, T.; Mukai, T.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2005, 127, 16396. (17) Klein, S. M.; Manoharan, V. N.; Pine, D. J.; Lange, F. F. Langmuir 2005, 21, 6669. (18) Sakatani, Y.; Boissiere, C.; Grosso, D.; Nicole, L.; Soler-Illia, G. J. A. A.; Sanchez, C. Chem. Mater. 2008, 20, 1049. (19) Englert, B. C.; Scholz, S.; Leech, P. J.; Srinivasarao, M.; Bunz, U. H. F. Chem.;Eur. J. 2005, 11, 995.

Published on Web 06/04/2009

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Article overnight prior to the measurement. The specific surface area was calculated by the BET equation, using the data in a P/P0 range between 0.05 and 0.2. Photoelectrochemical Experiments. All photoelectrochemical experiments were performed at 23 °C in a threeelectrode electrochemical cell with a quartz window for illumination. The resultant TiO2 film was sealed into a special electrode holder with 0.65 cm2 left unsealed for illumination and photoelectrochemical reaction.20 The assembled electrode was used as the photoanode (working electrode). A saturated Ag/AgCl electrode and a platinum mesh were used as reference and auxiliary electrodes, respectively. A voltammograph (CV27, BAS) was used for the application of potential bias. Potential and current signals were recorded using a Maclab A-D converter (AD Instruments). Illumination was carried out using a 150 W xenon arc lamp light source with focusing lenses (HF-200W-95, Beijjing Optical Instruments). To avoid the sample solution being overheated by infrared, the light beam was passed through an UV-band-pass filter (UG 5, Avotronics Pty. Ltd.) prior to illumination on the electrode surface. The light intensity was regulated and carefully measured at 365 nm.

Figure 1. Schematic illustration of formation and transformation of an organic/inorganic hybrid film into a pure inorganic film.

Experimental Section Chemicals and Materials. Indium tin oxide (ITO) conducting glass sheets were supplied by Delta Technologies Limited and were used as the substrate. Polystyrene monocarboxyterminated (PS, MW = 30 000) and titanium tetraisopropoxide (TTIP, 97%) were purchased from Science Polymer Inc. and Sigma-Aldrich, respectively. All other chemicals used were of analytical grade purchased from Sigma-Aldrich unless otherwise stated. Preparation of PS/TTIP Precursor Film. Typical precursor solution was prepared by dissolving 50 mg of PS and 26.0 μL (25 mg) of TTIP in 5.0 mL of chloroform in a sealed flask and then subjected to ultrasonic treatment until a transparent solution was obtained. A precleaned ITO substrate with an area of 177 mm2 defined by a spacer (15 mm in diameter) was placed onto a leveled sample holder in a homemade chamber at 23 °C. A typical PS/TTIP precursor film was prepared by casting 20 μL of the precursor solution onto the center of the defined area while a N2 stream saturated with 83% relative humidity was vertically blown to the surface at a flow rate of 400 mL/min. The precursor film was obtained when the complete solidification was achieved. Vapor Phase Hydrothermal Treatment. The VPH treatment was carried out in a sealed autoclave. The as-prepared precursor film was placed on a holder in an autoclave filled with a sufficient amount of deionized water. The holder was positioned 1.0 cm above the water level. The sealed autoclave was then placed in an oven at 100 °C for 72 h to complete the treatment. Conversion of VPH-Treated Precursor Film to Pure TiO2 Film. The VPH-treated films were transformed into pure TiO2

films via a calcination process, typically at 550 °C for 2 h. Characterization. SEM and TEM images of the samples were obtained using a JEOL JSM-6400F field emission scanning electron microscopy and FEI Tecnai 20 transmission electron microscopy, respectively. XRD patterns of the samples were collected using a Shimadzu XRD-6000 diffractometer, equipped with a graphite monochromator. Cu KR radiation (λ = 1.5418 A˚) and a fixed power source (40 kV and 40 mA) were used. N2 adsorption/desorption isotherms of the samples were obtained on a Quantachrome Autosorb-1 surface area and pore size analyzer. The samples were degassed at 150 °C in a vacuum below 10-3 Torr Langmuir 2009, 25(18), 11032–11037

Results and Discussion Figure 2 shows SEM images of a pure PS film and an asprepared precursor hybrid film. Shown in Figure 2a is a typical honeycomb structured BFs pattern obtained from pure PS precursor solution. It reveals a mean pore size of 3.3 μm with an average distance of 4.9 μm between the two neighboring pore centers. The images of the hybrid film (Figure 2b,c) display a highly ordered and long-ranged honeycomb structure similar to those BFs patterns of pure PS (see Figure 2a) and of other polymer films.21-27 The pore size was found to be uniform, having a mean value of 4.5 μm in diameter, which is significantly larger than that of pure PS films. The distance between the two neighboring pore centers was also found to be uniform, having an average value of 4.8 μm, which is similar to the value measured from the pure PS film shown in Figure 2a. A defect-free periodic honeycomb structured film was found to occupy the entire casting area (177 mm2) defined by the spacer, except at locations close to the edge of the spacer. The PS/TTIP hybrid film exhibits typical BFs patterns because it shares the same formation mechanism as pure organic films. Although PS acts as the structure-directing agent to define the shape and distribution of the BFs patterns, TTIP does play an important role to affect the film formation criteria and dimensional parameters of the BFs patterns. At the beginning of the BFs process, both PS and TTIP molecules are evenly distributed in the precursor solution. During the BFs formation process, PS self-assembly occurs as soon as the water condensation takes place (caused by the dramatic temperature drop induced by the rapid evaporation of chloroform). Under such conditions, hydrolysis of TTIP simultaneously takes place and TTIP can be partially (see eq 1) or completely (see eq 2) hydrolyzed, (20) Zhao, H.; Jiang, D.; Zhang, S.; Catterall, K.; John, R. Anal. Chem. 2004, 76, 155. (21) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79. (22) Park, M. S.; Kim, J. K. Langmuir 2004, 20, 5347. (23) Peng, J.; Han, Y.; Yang, Y.; Li, B. Polymer 2004, 45, 447. (24) Song, L.; Bly, R. K.; Wilson, J. N.; Bakbak, S.; Park, J. O.; Srinivasarao, M.; Bunz, U. H. F. Adv. Mater. 2004, 16, 115. (25) Widawski, G.; Rawiso, M.; Francois, B. Nature (London) 1994, 369, 387. (26) Yunus, S.; Delcorte, A.; Poleunis, C.; Bertrand, P.; Bolognesi, A.; Botta, C. Adv. Funct. Mater. 2007, 17, 1079. (27) Bunz, U. H. F. Adv. Mater. 2006, 18, 973.

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Figure 2. SEM images of as prepared precursor: (a) pure PS film, (b) PS/TTIP film (low magnification), and (c) PS/TTIP film (high magnification).

depending on the water availability:28-31 Ti½OCHðCH3 Þ2 4 þ 2H2 O f TiðOHÞ2 ½OCHðCH3 Þ2 2 þ 2HOCHðCH3 Þ2 Ti½OCHðCH3 Þ2 4 þ 4H2 O f TiðOHÞ4 þ 4HOCHðCH3 Þ2

ð1Þ

ð2Þ

These reactions occur mainly at water abundant sites/locations, such as the interfaces between moisturized N2 gas and the precursor solution and/or between water droplets and the precursor solution. The hydrolization products of TTIP are hydrophilic and can be accumulated at these water abundant interfaces, leading to an uneven distribution of titanium species. In addition, the hydrolization products of TTIP may be attracted to the hydrophilic end of PS (carboxyl group). These hydrophilic ends will be orientated toward the water abundant interfaces/sites during the self-assembly process, which could direct more TTIP hydrolyzed products to such places. As a result, the water abundant interfaces/sites should be rich of titanium source when film solidification is completed. This reflects on the resultant structure observed is the thick top walls (frames) around the pores (see Figure 2c). The effect of TTIP on the resultant structure could also be attributed to partial condensation of TTIP hydrolization products. The condensation/polymerization reactions (see eqs 3 and 4) can lead to the formation of titania network/clusters via Ti-oxo bridges,29 altering the PS arrangement. nTiðOHÞ2 ½OCHðCH3 Þ2  f ½ðCH3 Þ2 CHO2 ðOÞ-Ti -½O-Ti½OCHðCH3 Þ2 2 n -1 þ nH2 O

ð3Þ

nTiðOHÞ4 f ðOHÞ2 ðOÞ-Ti-½O-TiðOHÞ2 n -1 þ nH2 O ð4Þ Detailed investigations revealed that the critical criteria for the formation of a defect-free periodic honeycomb structured PS/ TTIP hybrid films are (i) the concentration of PS in the precursor solution is greater than 5.0 mg/mL, (ii) the ratio (w/w) between PS and TTIP must be greater than 1:1, and (iii) the N2 flow rate needs to be controlled between 100 and 500 mL/min, with a relative humidity greater than 70%. The resultant pore sizes can vary from 3.5 to 8.0 μm, depending on the above parameters. It should be mentioned that the effect of BFs experimental parameters on the pore size of PS/TTIP hybrid films differs from that of the (28) Soler-Illia, G. J. d. A. A.; Scolan, E.; Louis, A.; Albouy, P.-A.; Sanchez, C. New J. Chem. 2001, 25, 156. (29) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (30) Bischoff, B. L.; Anderson, M. A. Chem. Mater. 1995, 7, 1772. (31) O’Brien, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085.

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Figure 3. SEM image of the calcined PS/TTIP precursor film at 550 °C without vapor phase hydrolysis treatment.

formation of pure PS films. Adding TTIP into the precursor solution results in larger pore sizes in comparison to a pure PS film obtained using the same PS concentration precursor solution. Nevertheless, for a given PS concentration, a decrease in the ratio of PS/TTIP leads to a decrease in the pore size, although the resultant pores of the hybrid films are still larger than that of pure PS films. For pure PS film formation, an increase in the moisturized N2 flow rate normally leads to a decrease in the pore size, while the pore size of PS/TTIP hybrid films was found to be almost insensitive to flow rate changes within the range of 100-500 mL/min. The conversion of a PS/TTIP hybrid film into a pure TiO2 film was initially conducted by direct calcination of the as-prepared precursor PS/TTIP hybrid film at 550 °C. Figure 3 shows SEM image of the resultant film. It can be seen that the BFs templated honeycomb structure was completely destroyed as a result of the pyrolysis. A detailed investigation revealed that the deformation of the original hybrid honeycomb structure occurred whenever the applied calcination temperature was greater than the glass transition temperature of PS. It is therefore assumed that the honeycomb structure was dismantled by the high-mobility “liquefied” PS, formed during high-temperature pyrolysis process. The completely collapsed 3D structure suggests that the mechanical strength of the titania network/clusters formed during BFs process was poor and incapable of supporting titanium species in their original position, so preserving the original structure in the high-mobility “liquefied” PS, which is akin to the situation of soaking a sand castle in water. An attempt was made to reduce the impact of high-mobility “liquefied” PS by employing UV pretreatment. The as-prepared precursor PS/TTIP hybrid film was subjected to UV irradiation by a 500 W mercury light source. It was expected to completely photodecompose PS without involving a “liquefaction” process. Moreover, the impact of the high-mobility “liquefied” PS may be Langmuir 2009, 25(18), 11032–11037

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Figure 4. SEM images of the PS/TTIP precursor film after 24 h UV light treatment (a) and then calcined at 550 °C (b).

reduced even if a partial UV degradation of PS could be achieved. This is because the degradation products would have different molecular weights and hence different glass transition temperatures. The “liquefaction” of such a pretreated sample should occur over a much wider temperature range when it is subjected to a pyrolysis process. This may avoid the formation of large amount of high-mobility “liquefied” PS within a narrow temperature window and hence reduces the impact on the 3D honeycomb structure. The SEM image shown in Figure 4a indicates that the 3D honeycomb structure was reduced to a nearly 2D structure, as a result of UV degradation of organic components. This suggests that the mechanical strength of the titania network/clusters formed during the BFs process was not able to support the 3D structure even in the absence of high-mobility “liquefied” PS. It was found that thermal treatment of the UV-treated sample further reduced the structure to an almost flat hexagonal pattern, as a result of organic component being removed (Figure 4b). These results reaffirm that the mechanical strength of the titania network must be dramatically improved to enable the preservation of the BFs templated 3D structure during the pyrolysis process. A vapor phase hydrothermal method was therefore proposed to achieve better mechanical strength via the formation of Ti-oxo bridged large titania inorganic polymer networks. The VPH treatment was carried out at 100 °C in a sealed autoclave reactor with a holder designed to keep the sample above the level of water in the reactor. Under such conditions, the conversion of vast majority of TTIP into its fully hydrolyzed product (i.e., Ti(OH)4) might be anticipated. These favorable reaction conditions could also lead to high degrees of condensation/polymerization of hydrolyzed TTIP products to produce H2TixO1+x 3 nH2O or TixO2x 3 mH2O, forming stronger Ti-oxo type networks.28-31 Figure 5 shows the SEM images of the VPH-treated sample. The obtained structure was almost identical to the structure of the as-prepared precursor film. XRD patterns of the resultant film confirm the coexistence of amorphous and crystallized titania. This partial crystallization is a good indication of titania network formation, which was further confirmed by the FT-IR spectrum of the resultant film. A broad absorption band in the vicinity of 400-850 cm-1 and a relatively sharp absorption peak observed at 452 cm-1 confirm the presence of the Ti-oxo bridges.32-34 Interestingly, a strong broad absorption band in the vicinity of 800-1250 cm-1 and peaked at 1028 cm-1 can be assigned to TiO-C vibration,32 suggesting PS may also be involved in the network formation. However, the effectiveness of the proposed VPH method can be ultimately confirmed only when the preservation of the 3D structure of the hybrid film against the pyrolysis is achieved. Shown in Figure 6a-d are SEM images (32) Hu, Y.; Ge, J.; Sun, Y.; Zhang, T.; Yin, Y. Nano Lett. 2007, 7, 1832. (33) Tao, W.; Fei, F.; Wang, Y.-C. Polym. Bull. 2006, 56, 413. (34) Liufu, S.; Xiao, H.; Li, Y. J. Colloid Interface Sci. 2005, 281, 155.

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Figure 5. SEM images of PS/TTIP precursor film after vapor phase hydrolysis treatment at 100% relative humidity and 100 °C for 72 h: (a) SEM image of the resultant 3D honeycomb architecture; (b) SEM image of honeycomb structural frame.

Figure 6. SEM and HRTEM images and SAED patterns of the vapor phase hydrolysis treated samples after calcination at 550° for 2 h: (a, b) large-scale SEM image of the resultant pure TiO2 film; (c) SEM image of the resultant 3D honeycomb architecture; (d) honeycomb structural frame; (e, f) HRTEM image and SAED patterns of the resultant film.

of the VPH-treated sample calcined at 550 °C for 2 h. TG and FTIR data confirmed that the resultant film was a pure inorganic film and that the organic components of the hybrid film were completely removed during the calcination process. It can be seen that the originally templated 3D honeycomb structure was perfectly preserved across the entire film after the complete removal of all organic components via the pyrolysis process. This confirms that the Ti-oxo type networks formed during the VPH treatment possesses sufficient mechanical strength to hold the original 3D structure intact during the subsequent pyrolysis process. It should be noted that although the 3D honeycomb structure was perfectly preserved, the texture of the calcined sample was found to differ remarkably from that of a DOI: 10.1021/la901338j

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Figure 7. (a) Voltammograms obtained in water containing 0.1 M NaNO3 at different light intensity: curve 1: in dark; the light intensity for curves 2-8 in turn were 1.0, 2.5, 3.5, 5.0, 6.6, 8.3, and 9.6 mW/cm2 (measured at 365 nm). (b) Saturation photocurrent and light intensity relationship.

VPH-treated hybrid film without pyrolysis. A VPH-treated hybrid film before calcination possesses a dense and smooth texture (see Figure 5b), while a pure inorganic film resulting from the calcination exhibits a rough surface and porous texture as a consequence of the organic contents being removed (Figure 6d). For a calcined TiO2 film, data obtained from N2 adsorption/ desorption isotherms reveal a BET surface area of 128 m2/g with a mean pore size of 20 nm and a pore volume of 0.771 cm2/g. Figure 6e shows a high-resolution TEM image of a typical calcined inorganic TiO2 film, having the lattice fringes clearly visible, signifying high crystallinity. This was further confirmed by the SAED patterns of the resultant film shown in Figure 6f. The mean crystallite size of 15 nm was also obtained from XRD patterns according to Scherrer’s equation. The phase composition of the inorganic film calcined at 550 °C was also determined by XRD as pure anatase phase with no trace of rutile or brookite. The photocatalytic activity of the resultant film was subsequently examined. The resultant inorganic 3D honeycomb structured TiO2 film was sealed into a special electrode holder with a defined area of 0.65 cm2 left open for UV illumination. The assembled electrode was used as the photoanode for photocatalytic oxidation of water and organic compounds. The photocatalytic oxidation of water was carried out in a solution containing 0.1 M NaNO3 supporting electrolyte under different light intensities. The obtained voltammograms shown in Figure 7a revealed that without illumination only a negligible dark current was measured (curve 1 in Figure 7a). For all cases with UV illumination, the photocurrents increase linearly as the applied potential bias increases within the low potential range. This can be attributed to the limitation of free photoelectron transport within 11036 DOI: 10.1021/la901338j

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Figure 8. (a) Voltammograms obtained from a 450 mM glucose solution containing 0.1 M NaNO3 at different light intensity: curve 1: in dark; the light intensity for curves 2-8 in turn were 1.0, 2.3, 3.3, 5.0, 6.6, 8.2, and 9.6 mW/cm2. (b) Net saturation photocurrent and light intensity relationship.

the TiO2 catalyst film.35,36 The photocurrents saturate at higher potentials due to the limitation of the photohole capture process at the catalyst/solution interface.35,36 These characteristics are qualitatively the same as those observed from a nanoparticulate TiO2 photoanode.35-37 The magnitude of the saturated photocurrents (Isph) can be used to quantitatively represent the photocatalytic activity of a photocatalyst as it represents the maximum rate of oxidation under a given light intensity.35,37 A plot of the saturation photocurrent against the light intensity gives a straight line, having a slope of 10.636 μA/mW and R2 = 0.9992, as shown in Figure 7b. This slope value was found to be over 35% higher than that obtained from a nanoparticulate TiO2 photoanode with the same TiO2 loading per unit area, implying an improved photocatalytic activity toward water oxidation. The photocatalytic oxidation of organics was carried out in a 450 mM glucose solution containing 0.1 M NaNO3 under different light intensities. The obtained voltammograms shown in Figure 8a revealed the same characteristics as shown in Figure 7a for water oxidation, except that the measured Isph was almost 10-fold of that obtained for water oxidation due to the presence of organics. Under such circumstances, the measured photocurrent consists of two components, which are attributable to photocatalytic oxidation of water and organics.20 The net saturation photocurrent (ΔIsph), due purely to the photocatalytic oxidation of organics, can be obtained by subtracting the blank saturation photocurrent (originated from water oxidation) from the overall saturation (35) Jiang, D.; Zhao, H.; Zhang, S.; John, R. J. Phys. Chem. B 2003, 107, 12774. (36) Jiang, D.; Zhao, H.; Zhang, S.; John, R. J. Catal. 2004, 223, 212. (37) Jiang, D.; Zhang, S.; Zhao, H. Environ. Sci. Technol. 2007, 41, 303.

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photocurrent (originated from oxidation of both water and organics).20 Plotting the ΔIsph vs the light intensity also gives a linear relationship, having a slope value of 108.728 μA/mW and R2 = 0.9983 (Figure 8b). This slope value is nearly 30% higher than that obtained from a nanoparticulate TiO2 photoanode with the same TiO2 loading, which confirms an improved photocatalytic activity toward oxidation of organics.

Conclusions In summary, we have demonstrated a simple and effective vapor phase hydrothermal modification method capable of transforming a 3D honeycomb structured PS/TTIP hybrid film into a photoactive TiO2 film without dismantling the originally

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templated 3D structure. The resultant film can be directly used for photocatalysis applications and could be further modified for other applications. In principle, this method can be used to preserve 3D structures of other organic/inorganic hybrid films during their conversion to pure inorganic films by means of pyrolysis, if mechanically strong inorganic polymer networks can be formed via hydrolysis reactions, under hydrothermal conditions. The ability to preserve the preferred 3D structure during the subsequent conversion processes enables the realization of full benefit of unique architectures created by a templating method. Acknowledgment. This work is supported by the Australian Research Council.

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