Environ. Sci. Technol. 2009 43, 9425–9431
Hierarchically Macro-/Mesoporous Ti-Si Oxides Photonic Crystal with Highly Efficient Photocatalytic Capability J I A N L I U , †,‡ M I N G Z H U L I , * ,† J I N G X I A W A N G , † Y A N L I N S O N G , * ,† LEI JIANG,† TAKETOSHI MURAKAMI,§ A N D A K I R A F U J I S H I M A * ,§ Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China and Kanagawa Academy of Science and Technology, KSP Building West 614, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan
Received August 12, 2009. Revised manuscript received September 16, 2009. Accepted October 26, 2009.
Hierarchically macro-/mesoporous Ti-Si oxides photonic crystal (i-Ti-Si PC) with highly efficient photocatalytic activity has been synthesized by combining colloidal crystal template and amphiphilic triblock copolymer. It was found that the thermal stability of mesoporous structures in the composite matrix were improved due to the introduction of silica acting as glue and linking anatase nanoparticles together, and the photocatalytic activity of the i-Ti-Si PCs was affected by the calcination conditions. The influences of photonic and structural effect of the i-Ti-Si PCs on photocatalytic activity were investigated. Photodegradation efficiency of the i-Ti-Si PCs was 2.1 times higher than that of TiO2 photonic crystals (i-TiO2 PCs) in the photodegradation of Rhodamine B (RB) dye as a result of higher surface area. When the energy of slow photon (SP) was optimized to the abosorption region of TiO2, a maximum enhanced factor of 15.6 was achieved in comparison to nanocrystalline TiO2 films (nc-TiO2), which originated from the synergetic effect of SP enhancement and high surface area.
Introduction Titania photocatalysis has been proven to be a promising method for the purification and treatment of contaminated air and wastewater (1, 2). For practical applications, enormous efforts have been devoted to improve the photocatalytic activity of TiO2, including coupling with other oxides, transition metal and nonmetal doping, surface modifications, and so on (3-7). Despite the achievements in this field, the TiO2 photocatalysis still suffers from low photocatalytic efficiency due to the low surface area and the weak light harvesting efficiency (8). Recently, hierarchically macro-/mesoporous TiO2 materials have aroused extensive attention in photocatalytic field * Address correspondence to either author. Phone/fax: +86-1062529284 (Y.S.). E-mail:
[email protected] (Y.S.); fujishima@ newkast.or.jp (A.F.);
[email protected] (M.L.). † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences. § Kanagawa Academy of Science and Technology. 10.1021/es902462c CCC: $40.75
Published on Web 11/05/2009
2009 American Chemical Society
(9-15). High surface area of the mesoporous structure and three-dimensionally (3D) interconnected architecture contribute to high photocatalytic efficiency. However, efforts to improve TiO2 photocatalytic efficiency by improving light harvesting were quite limited (11). Photonic crystals (PCs) with ordered macroporous structure give rise to the photonic stop band for certain frequencies of light (16, 17). The application of PC-based ordered macroporous photocatalyst is promising, as photons near photonic stop band edges can be slowed down (termed slow photon, abbreviated as SP) and localized in the active material to increase the photocarrier generation efficiency. So, constructing TiO2 into PC structure can promote light absorbance of TiO2, which has been demonstrated in photocatalytic process (18-22). Furthermore, incorporating mesoporous structure into PC framework would maximize the use of photogenerated electron-hole pairs for photocatalytic reactions due to the more active sites of mosopores (23, 24). Hence, a photocatalytic film containing mesoporous framework as well as 3D ordered macropores is desirable for photocatalysis, which will combine high surface area with efficient light harvesting. However, the mesostructured TiO2 suffers from collapse of mesopores due to its poor thermal stability, thereby losing partial surface area (11, 25-28). Therefore, it is critical to overcome the thermal instability problem in order to acquire hierarchically ordered macro-/mesoporous photocatalysis system. In the present work, the hierarchically macro-/mesoporous Ti-Si oxides PC (i-Ti-Si PC) was prepared through the double-template synthesis combining colloidal crystal and amphiphilic triblock copolymer. Toward optimal photocatalytic performance, each synthesis procedure was optimized including the precursor selection, infiltration step, and template removal conditions. By adopting colloidal template-assisted assembly (29), a series of i-Ti-Si PCs with different stop bands were prepared by controlling the sizes of colloidal crystal spheres. Through adopting the “brick and mortar” strategy (25), the thermal stability of mesostructures was improved greatly due to the glue effect of silica in the TiO2 nanoparticles matrix (27, 28). Scanning electron microscope (SEM), Transmission electron microscope (TEM) and X-ray diffraction (XRD) were employed to characterize 3D ordered macroporous structures, the thermal stability of mesostructures and anatase crystallinity in the PC matrix upon calcinations at different conditions, repectively. The results demonstrated that the i-Ti-Si PCs upon calcination at 650 °C could retain the mesostructures in the ordered macroporous skeleton and had good anatase crystallinity. Photonic and structural effects on photocatalytic activity were investigated in the photodegradation of Rhodamine B dye (RB) under monochromatic light illumination. Photodegradation efficiency of the i-Ti-Si PCs was 2.1 times higher than that of the i-TiO2 PCs due to the higher surface area. When the energy of SP was tuned into the absorption region of anatase TiO2, the absorbance of TiO2 in ultraviolet (UV) region was enhanced, which contributed to produce more electron-hole pairs for photocatalytic reaction. In this case, for i-Ti-Si PC, a maximum enhanced factor of 15.6 was achieved in comparison to nanocrystalline TiO2 films (nc-TiO2), which originated from the synergetic effect of SP enhancement and high surface area. This is the first example of the hierarchically ordered Ti-Si oxides PCs applied in the photocatalytic field and the results illustrated the potential of the i-Ti-Si PC as a highly efficient photocatalyst. VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 1. Diagram of procedure for preparing the 3D ordered macro-/mesoporous i-Ti-Si PCs
Experimental Section Materials. Titanium isopropoxide (TIPO), tetraethyl orthodilicate (TEOS), hydrochloric acid, acetic acid, ethyl alcohol were bought from Wako. Tetrabutyl titanate (TBT) and the amphiphilic triblock copolymer Pluronic P123 [HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H((EO)20(PO)70(EO)20, abbreviated as P123] were purchased from Aldrich. Ultra pure water (18.3 MΩ/cm) was used in all the experiments. Fabrication of Colloidal Crystal Templates. Monodisperse core-shell P(St-MMA-AA) colloidal spheres were synthesized by emulsifier-free batch emulsion polymerization using a modified method (30). The glass or quartz slides were cleaned in freshly prepared Piranha solution (H2SO4/H2O2, 3:1), and rinsed repeatedly with water. To deposit colloidal crystal templates on clean substrates, the slides were positioned vertically in a vial containing the monodisperse P(St-MMA-AA) aqueous suspension of 0.25 wt % at invariant temperature (60 °C) and humidity (60%) for 24 h (29). Prior to infiltration, colloidal crystal templates were annealed at 85 °C for half a hour to enhance the bond among colloidal spheres (31). Fabrication of the i-Ti-Si PC. In a typical synthesis (28), 1.0 g of P123 was dissolved in 30 g of ethyl alcohol, followed with slowly adding 1.8 g of hydrochloric acid with vigorous stirring. The reaction vessel was kept stirring for 3 h at 40 °C, then 2.34 g of TIPO and 0.434 g of TEOS were added with vigorous stirring for another 5 h at 40 °C. The obtained colorless transparent solution was allowed to undergo hydrolysis reaction at room temperature overnight. The PCs with photonic stop band maximum at 350, 365, 386, and 512 nm were prepared by dipping the colloidal crystal templates with sphere diameters of 240, 270, 300, and 420 nm into Ti-Si oxides alkoxide precursor sol. Infiltration was performed by placing the templates in precursor solution at 45 °C for 10 min. The alkoxide precursor infiltrated the voids of template through capillary force and hydrolyzed there. Nanocrystalline films (nc-Ti-Si) were prepared on ascleaned glass slides by spin-coating of alkoxide precursor sol at 3000 rpm for four times. All the films were dried in an oven at 80 °C overnight followed by calcination at 450-650 °C between 4 and 24 h at a ramp rate of 1 °C /min. Fabrication of the i-TiO2 PC. In a typical synthesis, 20 g of TBT was dissolved in 20 g of ethyl alcohol with stirring for 1 h. Then 2.12 g of acetic acid, 4.06 g of hydrochloric acid and 7 g of ethyl alcohol were mixed homogenously and followed with adding dropwize into the above solution and stirred for another 3 h. The obtained yellow transparent solution was allowed to undergo hydrolysis reaction at room temperature overnight. The infiltration process was identical to that of the i-Ti-Si PC. The as-dried films were annealed at 450 °C for 4 h at a ramp rate of 1 °C /min. 9426
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Materials Characterization. SEM images were taken by JEOL FE-SEM 6700F microscopy operating at 3.0 kV. TEM images were obtained on a JEOL JEM-2010 transmission electron microscope operating at 200 kV. Samples obtained by scratching the films from the substrate for TEM measurements were dispersed in ethanol. Carbon coated copper grids were used as the sample holder. XRD patterns were collected on a Rigaku D/MAX 2500 X-ray powder diffractometer using a high-power Cu KR radiation. Reflective spectra were retrieved from Shimadzu UV-2450 UV-visible spectrophotometer. Depth-sensing indentation experiments were performed using MTS Nano Indenter XP with a diamond indentation tip. Eight independent indentations for one sample were performed at maximum indentation depth (2 µm) and the optimum results were compared with each other. Nitrogen adsorption-desorption isotherms were obtained on a Micromeritics ASAP 2020 apparatus at -196 °C. Prior to the measurements, the samples were degassed at 250 °C for 10 h. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. The pore size distribution was derived from the adsorption branches of the isotherms using the Barrett-Joyner-Halen (BJH) method. Photocatalytic Testing. In the solid state photocatalytic process, 40 µL 0.1 mmol/L RB in ethanol was dropped onto the film followed with placed in the dark for 10 min. The photocatalytic activity was evaluated by the RB dye decomposition efficiency under monochromatic light irradiation (365 ( A5 nm) from Hayashi LA-310 UV lamp equipped with Oriel 74000 monochromator. The radiant flux was measured with a photometer (International Light model). After illumination, the PCs were fixed onto the sample holder of the Shimadazu UV-2450 UV-visible spectrophotometer and the extinction spectra were acquired after subtracting the film background at interval during the total time course for 15 min∼1 h. The photodegradation experiments for each sample were performed three times independently and the photodegradation rate constants were averaged over three times.
Results and Discussion Fabrication and Characterization of the i-Ti-Si PCs. Scheme 1presents the synthesis diagram of the hierarchically macro-/mesoporous i-Ti-Si PC, which was obtained by combining the colloidal crystal template and the assembly of TIPO and TEOS with amphiphilic triblock copolymer. Under strong acidity condition, the above soluble inorganic species and triblock copolymer combined to form mesostructured hybrid intermediates in the voids of the colloidal crystal template (32). Colloidal crystal assembled from monodisperse P(St-MMA-AA) colloidal spheres were used as the template for 3D ordered macropores (30, 33); mesopores in the framework of macroporous skeleton were derived
FIGURE 1. Typical SEM and TEM images of the i-Ti-Si PC (a, b, c) and the control sample of the i-TiO2PC (d). SEM images of the i-Ti-Si PC (a, b) showing the 3D hexagonally ordered macroporous structures. TEM image of the same PC (c) shows that the framework of the i-Ti-Si PC macroporous skeleton consists of mesopores with size ∼6 nm. TEM image of the i-TiO2 PC (d) shows the framework consists of randomly oriented nanoparticles with sizes of 15-30 nm. from the amphiphilic triblock copolymer (34). After calcined to remove the templates, the hierarchically macro/mesoporous i-Ti-Si PC was obtained. Figure 1a shows the typical images of the as-preapred i-Ti-Si PC compared with the control sample of the i-TiO2 PC. The SEM and TEM images show that 3D ordered macropores are formed. The PCs were derived from the colloidal crystal with the P(St-MMA-AA) spheres of 420 nm in diameter. The size of hexagonally arrayed macropores in Figure 1b is smaller than that of their original template, which was due to the decomposition and vaporization of the colloidal crystal template during calcination processes (35, 36). From TEM images, it can be clearly observed that the i-Ti-Si PC consists of mesopores with size of ∼6 nm, and the framework of the i-TiO2 PC consists of randomly oriented nanoparticles with sizes of 15-30 nm (Figure 1c and d). The width of framework wall of the i-Ti-Si PC is much broader than that of the i-TiO2 PC, indictaing a higher filling fraction. In addition, the highly ordered strucutre of the i-Ti-Si PC spreads over larger area than the control sample of the i-TiO2 PC (See Supporting Information (SI) Figure S1). Thermal Stability and Mechanical Strength of the i-Ti-Si PCs. Usually, mesostructured TiO2 suffers from the collapse of mesopores due to an amorphous-crystalline phase transition during heating (25, 27). Various efforts have been devoted to improve the thermal stability, including heat treatment, increasing the thickness of mesostructured wall, or adding some amorphous component into the TiO2 framework (25, 28, 37, 38). In our work, a thermally stable hierarchically ordered macro-/mesoporous Ti-Si oxides composite PC was synthesized. Under the circumstances, silica from TEOS plays the role of mortar and titania nanoparticles from TIPO act as the brick to construct thermally stable mesostructures.
Figure 2 presents TEM images of the i-Ti-Si PCs upon calcination at four conditions: (a) 450 °C, 10 h; (b) 450 °C, 24 h; (c) 550 °C, 6 h; (d) 650 °C, 4 h. Figure 2a and c clearly demonstrate the mesopores were retained in the macroporous skeleton upon calcination at 450-550 °C between 4 and 6 h. But as illustrated in Figure 2b, upon calcination for long time (24 h), the anatase TiO2 nanoparticles in the PC matrix will grow too large to retain the mesostructures. Figure 2d clearly shows that the mesopores survived in the macrostructured skeleton during the high temperature calcinations even up to 650 °C. Figure 3a gives XRD patterns of the Ti-Si oxides precursor sol powder upon above calcination conditions. It suggests a gradual intensity increasing and narrowing in the anatase TiO2 (101) peaks (2θ ) 25.4°) with increasing calcination temperature. The well-resovled peaks of anatase TiO2 indicate the transition from amorphous to anatase phase and the formation of highly crystalline mesoporous framework in the macroporous skeleton (28, 39). As expected, the calcinating conditions affected the photocatalytic activity of the i-Ti-Si PCs in the photodegradation process of the RB dye (39), as illustrated in Figure 3b. For practical photocatalytic applications, the mechanical strength of the photocatalytic films was also an important factor in determining whether the photocatalytic system could endure numerous usages. Therefore, depth-sensing indentation measurement was employed to characterize mechanical properties of the i-Ti-Si PCs upon calcination at different conditions (40). The results displayed that the crushing pressures of the i-Ti-Si PC structures were proportional to increment of anatase crystallinity in the Ti-Si oxide matrix and the i-Ti-Si PC calcinated at 650 °C exhibited higher mechanical strength than i-TiO2 PC due to the good crystallinity and higher filling fraction (see SI Figure S3). Therefore, the highly crystalline TiO2 within 3D ordered macro-/mesoporous PC matrix, in which mesopores surVOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. TEM images of the i-Ti-Si PCs subjecting to different calcination conditions: (a) 450 °C, 10 h; (b) 450 °C, 24 h; (c) 550 °C, 6 h; (d) 650 °C, 4 h.
FIGURE 3. (a) XRD patterns of Ti-Si oxides precursor sol powders and (b) photodegradation kinetics of the RB dye adsorbed on the four i-Ti-Si PCs with same stop bands subjecting to different calcination conditions. vived after 650 °C calcination due to the glue effect of silica, was obtained and would be beneficial for the photocatalytic process. 9428
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Photocatalytic Performance of the i-Ti-Si PCs. In our photocatalytic system, to investigate the role of the stop band reflection, the SP enhancement and multiple scatterings on the photocatalytic activity respectively, monochromatic light (365 ( 5 nm) corresponding to the excitation wavelength of TiO2 was adopted. Four stop bands were selected according to the illumination window (as illustrated by the shaded area in Figure 4a), which were categorized into four types: (1) 365 nm, stop band coinciding with the illumination; (2) 350 nm, the SP localized in the high dielectric part of the material coinciding with the illumination; (3) 386 nm, the SP localized in the low dielectric part of the material coinciding with the illumination; (4) 512 or 600 nm, the SP was far away from the illumination. Varying the stop bands of PCs can make SP wavelength move into and out of the illumination window. Nanocrystalline films without photonic effect were used as the references. It has been demonstrated that the light with photon energy at both sides of photonic stop band can be described as a standing wave (41). The peaks of light waves are localized in different parts of the matrix, depending on the photon energy. For photocatalytic applications, PC serves to promote light absorption of TiO2 through increasing interaction time between photon and TiO2 (20, 22). When light wavelength locates on the red side of PC, the SP effect is able to enhance the absorbance of TiO2 in UV region, thereby producing more electron-hole pairs and leading to enhanced photodegradation efficiency (Figure 4b). The photocatalytic activity of the i-Ti-Si PCs and the i-TiO2 PCs was evaluated by the photodegradation efficiency of the RB dye adsorbed on the films under 365 nm light irradiation (20, 21). The experiment was conducted in the solid state to avoid the interfering factors usually involved in heterogeneous photocatalytic process such as diffusion of dye molecules and convection of solvent (2). The photodegradation kinetics was measured in ambient condition, through monitoring the decay of the absorbance maximum of the RB dye adsorbed on the films, as a function of the
FIGURE 5. Photodegradation kinetics of the RB dye under 365 nm monochromatic light irradiation: logarithmic plot of dye extinction as a function of irradiation time for (a) the i-Ti-Si PCs with stop bands centered at 350 nm, 365, 386, and 512 nm; (b) the i-TiO2 PCs with stop bands centered at 350 nm, 365, 386, and 600 nm. Nanocrystalline films (nc-Ti-Si and nc-TiO2) and mesoporous SiO2films were used as the references. FIGURE 4. (a) Reflectance spectra of the i-Ti-Si PCs with stop bands centered at 350, 365, 386 nm, and 512 nm. Labeled numbers indicate the photonic stop band positions of the PCs. The spectra are displaced vertically for clarity. The shaded area represents 365 nm monochromatic light irradiations. (b) The schematic photoexcitation process of TiO2 for photodegradation of the RB dye under 365 nm light irradiation. At photon energies near the photonic stop band from the red side, light can be described as a standing wave with peaks localized in the high dielectric part of PC, i.e., TiO2 part. irradiation time (as illustrated in SI Figure S4). For comparisons, all the films used herein were of the same size and loaded with the same amount of the RB dye with only difference in the stop band positions (21). Figure 5a shows that the linear correlation of time profile of ln(/0) at the initial reaction stage, which suggested the pseudo first-order reaction kinetics for the i-Ti-Si PCs in the photodegradation of the RB dye. At the same time, a series of the i-TiO2 PCs with four stop bands corresponding to the i-Ti-Si PC was performed under the same conditions and the linear correlation also suggests a first-order reaction for the i-TiO2 PCs, as shown in Figure 5b. Two nanocrystalline films (the nc-Ti-Si and the nc-TiO2) without photonic stop band and mesoporous SiO2 films as references were also conducted in the photodegradation experiment to evaluate the photonic effect on the photodegradation efficiency. The photocatalytic activity of the PCs can be quantitatively evaluated by comparing the apparent reaction rate constants (denoted as k), as illustrated in histograms of Figure 6. Direct comparison of the reaction rate constants, ki-Ti-Si is markedly improved than ki-TiO2 due to the higher surface area. As illustrated in SI Figure S5, the measured Brunauer-Emmett-
FIGURE 6. Comparison of the apparent reaction rate constants of the i-Ti-Si PCs and the i-TiO2 PCs with a series of stop bands centered at 350, 365, 386, and 512 nm (in this case, i-TiO2-600). The photodegradation rate constants were averaged over three times. Teller (BET) surface areas of the i-Ti-Si PCs and i-TiO2 PCs were 192.8 m2 g-1 and 56.6 m2 g-1, respectively. An enhanced factor, defined as the ratio of the photodegradation rate constant of i-Ti-Si PC relative to i-TiO2 PC and nanocrystalline films, was used to supply quantitative comparison of the photocatalytic activity. In the time course, the extinction of dye adsorbed on the mesoporous SiO2 films showed no change. Among all the PCs and nanocrystalline films, the i-Ti-Si-350 PC (the number suffix denotes the stop band maximum of PCs) gave the highest photodegradation efficiency, and enhanced factors of 2.0 and 15.6 in VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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comparison to the i-TiO2-350 PC and the nc-TiO2 were obtained, respectively (ki-Ti-Si ) 0.3993 min-1). For the i-Ti-Si-350 PC, the SP localized in the high dielectric part of the PC coincided with the illumination window, which contributed to strong anatase absorption and thereby produced more electron-hole pairs for photodegradation of the RB dye (20). Furthermore, the mesopores provided more active sites for photocatalytic reaction and would maximize the use of the photogenerated electron-hole pairs (23, 24). When the stop band moved from 350 to 365 nm, the photodegradation efficiency of the i-Ti-Si-365 PC (ki-Ti-Si ) 0.09071 min-1) became the least active one among all the i-Ti-Si PCs, but it was still 2.3 times higher than that of the i-TiO2-365 PC. Under the circumstances, the partial reflection accounted for the lower efficiency due to the stop band reflection (42). For PCs with stop band centered at 386 nm, the SP localized in the low dielectric air part of the materials coincided with the illumination window. The photodegradation rate of i-Ti-Si-386 (0.242 min-1) is 2.5 times higher than the i-TiO2-386 PC. High photodegradation rates relative to the nc-TiO2 should be ascribed to the localization of photon in the low dielectric of PC matrix and high surface area (41). While for the i-Ti-Si-512 PC, whose stop band position was far away from the illumination window, the photodegradation reaction rate constant of the i-Ti-Si512 PC was 1.6 times higher than that of the corresponding TiO2 PC. In this situation, the SP effect no longer worked and the high photocatalytic activity relative to the nc-TiO2 should come from random light scatterings, which was intrinsically the same property with the SP effect (43-45). Therefore, based on the above data and analysis, we concluded that the i-Ti-Si PCs were superior to the i-TiO2 PCs in photodegradation of the RB dye. The enhanced factor of 2.1 times in average was achieved due to high surface area of mesopores. Comparison of the nc-Ti-Si with the nc-TiO2 in photodegradation rate constants further supported the conclusion that the enhancement of photodegradation efficiency was ascribed to the mesopores. These results clearly pointed out that a maximum enhanced factor of 15.6 in comparison to the nc-TiO2 could be achieved by combining the synergic effect of SP enhancement and high surface area and illustrated the potential of the i-Ti-Si PC as a highly efficient photocatalyst. A further modification of noble metal to the i-Ti-Si PC is expected to more effectively enhance the photocatalytic performance.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 50625312, U0634004, 20721061 and 50973117), the 973 Program (No. 2006CB806200, 2006CB932100, 2006CB921706, 2007CB936403 and 2009CB930404), Chinese Academy of Sciences (No. KJCX2-YW-M11), and was also supported by a Grant-in-Aid for Scientific Research on Priority Area 417, the Asia S&T Strategic Cooperation Program from the Japanese Ministry of Education, Culture, Sports, Science and Technology. Jian Liu thanks Dr. Kazuya Nakata, Dr. Tsuyoshi Ochiai and Miss Nomura of KAST and Dr. Zhihong Ji of ICCAS for helpful discussions.
Note Added after ASAP Publication Due to a production error, this paper published ASAP November 5, 2009 with incorrect author affiliation designations; the correct version published ASAP on November 9, 2009.
Supporting Information Available SEM images of the i-Ti-Si PC and the i-TiO2 PC over large area, reflectance spectra of the i-TiO2 PC, mechanical strength comparisons of the i-Ti-Si PCs upon different 9430
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calcination conditions, typical extinction spectra of the RB dye as a function of illumination time and nitrogen adsorption-desorption isotherms and pore size distribution data. This material is available free of charge via the Internet at http://pubs.acs.org.
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