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Emulsion templating to obtain dual-size-scale mesoporous titania coatings
Materials Letters 63 (2009) 2619–2621
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Emulsion templating to obtain dual-size-scale mesoporous titania coatings Sarika Phadke ⁎, Jay Ho, Dunbar P. Birnie III Materials Science and Engineering, Rutgers University, Piscataway, New Jersey-08854, United States
a r t i c l e
i n f o
Article history: Received 21 August 2009 Accepted 7 September 2009 Available online 13 September 2009 Keywords: Emulsion templating Porosity Microstructure Solar energy materials
a b s t r a c t Meso- and macroporous TiO2 coatings have been fabricated using oil-in-water emulsions as the templating phase. Coatings with interpenetrating, polydisperse pore structures are obtained which are useful for applications such as dye sensitized solar cells (DSSC), where permeation of one or more reactants into the inner surface areas of the coating microstructure is critical. Very uniform, defect free coatings are obtained with commercially available TiO2 nanopowder, with pores in the range of 30–1000 nm. Characterization of the coatings was performed using mercury porosimetry and electron microscopy. © 2009 Published by Elsevier B.V.
1. Introduction Dye sensitized solar cells using nanocrystalline TiO2 coatings as the photoanode, are technically and economically strong competitors to the existing photovoltaic technologies [1–4]. During the dye sensitized solar cell operation, the dye molecules, adsorbed on the titania coating, get excited and inject electrons into the conduction band of the TiO2 particles. The nanometer sized, sintered TiO2 particles allow electronic conduction towards the transparent conductive oxide (TCO) and the dye molecule recovers back to its initial ground state by oxidizing the electrolyte. The TiO2 photoanode is supposed to perform the following four major functions in the solar cell: (1) provide surface for the dye adsorption, (2) accept electrons from the excited dye, (3) conduct electrons to the TCO, and (4) allow for regeneration of the dye by giving access to the electrolyte through the porous structure. In order to perform the first and the last functions as stated above, the titania film should possess meso- (2–50 nm sized pores) and macro- (>50 nm pores) porosity with interconnecting pore channels [5–9]. Several methods have been tried to obtain a porous structure of TiO2 nanoparticles including combination of sol–gel and templating techniques [10–17]. Jiu et al. [13] used copolymer F-127 (poly (ethylene oxide)106–poly(propylene oxide)70–poly(ethylene oxide)106) and surfactant CTAB (cetyltrimethylammonium bromide) to obtain templated growth of porous TiO2 films. A similar structure was obtained by Zukalová et al. [17] using Pluronic P-123. Although a high surface area was achieved in these works, the pore diameters obtained were quite small in the range of 4–8 nm which would affect the diffusion of redox species and penetration of organic hole conductor adversely. In the earlier work of our research group, we demonstrated a simple mixing–casting
⁎ Corresponding author. E-mail address: [email protected] (S. Phadke). 0167-577X/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.matlet.2009.09.021
method to obtain templated TiO2 coating with meso- and macroporosity and improved film quality [8]. In that work we had used monosized polystyrene particles to template the interconnected pore structures. Interestingly, we found that our best solar cell efficiency was achieved when the templates had created a percolating interconnected – but still disordered – network of large-size pore channels [9]. That observation paved the way for the present work where a random emulsion was used for templating. In this paper we report on this alternative, cost effective templating method using oil-in-water emulsions as the templating phase. In earlier work oil/water interface templating has been used to obtain acid-prepared mesostructures of silica on two different length scales [18]. Imhof and Pine [19–21] demonstrated use of emulsion templating in the sol–gel process to produce macroporous silica. Since most of the metal alkoxides, including titanium alkoxide, lead to immediate precipitation in the presence of an aqueous medium, Imhof and Pine used non-aqueous emulsions to obtain porous titania. In the present work, we have employed simple aqueous emulsions to incorporate meso- and macroporosity into titania coatings, starting with pre-synthesized titania dispersion. This method provides a much less expensive and simpler way to obtain dual porosity TiO2 coatings with uniform, crack free film quality.
Table 1 Particle size measurement of emulsions with different oil concentrations. Oil concentration (%)
Mean diameter — volume averaged (nm)
10 20 30 40
107 165 3900 5650
2620
S. Phadke et al. / Materials Letters 63 (2009) 2619–2621
were dried in air for 15 min and then annealed at 500 °C for 30 min with a ramping rate of 2°/min. Characterization: Brookhaven Instrument's ZetaPALS 90Plus particle size analyzer was used to measure the particle size distribution of the oil droplets created in the oil-in-water emulsions. The pore structure was analyzed using Micromeritics AutoPore 9400 mercury porosimeter and the appearance of the microstructure was studied using a Zeiss-DSM 982 scanning electron microscope. 3. Results and discussion
Fig. 1. Mercury porosimetry plots for non-templated and 20% emulsion templated coatings. Templated plot shows extra porosity sizes created by oil droplets during drying.
2. Experimental Synthesis: A stable oil-in-water emulsion was prepared using paraffin oil and Tween 80/Span 80 emulsifier blend obtained from Sigma-Aldrich. Titanium dioxide nanoparticles (Aeroxide-P25) were obtained from Evonik/Degussa. Aeroxide P25 contains 70% anatase and 30% rutile titania. Upon heat treatment above 450 °C mainly anatase phase exists [22].Oil-in-water emulsions were prepared with effective HLB (hydrophile–lypophile balance) of the emulsifier blend set to 11. The P25 titania nanoparticles were then dispersed in the emulsion using Hamilton Beach Scovill 936 homogenizer. 10 to 20 micron thick coatings were doctor bladed on fluorinated tin oxide (FTO) coated glass substrates using scotch tape to control the thickness. The coatings
Table 1 gives volume averaged diameters of the oil droplets as a function of composition for four mixing ratios: 10, 20, 30 and 40%. It was observed that the 30% and 40% emulsions started to sediment after 15–20 min, which was a result of much bigger oil droplet size, indicating that these had not been stabilized well enough for further testing. Therefore emulsions with 20% oil were used for further experiments discussed below. Upon dispersing titania nanoparticles into the 20% oil-in-water emulsion, a thick paste was obtained. This paste was doctor bladed onto cleaned FTO coated glass substrates. The thickness of the coating was controlled by two pieces of scotch tape. The coating was allowed to air dry slowly and then heat treated to 500 °C for 30 min. Typically, the final coating thickness, measured by profilometry, was around 15–20 µm with the templated solution and around 10–15 µm with the nontemplated solution. Some coatings were gently scrapped off the substrate to gather enough material for accurate mercury porosimetry. Fig. 1 shows the comparison of the results obtained with the mercury porosimetry, for non-templated and 20% emulsion templated samples. The non-templated titania sample showed 53% porosity while the 20% emulsion templated sample showed 69% porosity. The BET specific surface area on the other hand did not differ much. Both the samples had
Fig. 2. Microstructure evaluation with scanning electron microscopy: a) non-templated titania coating at 5000×, b) 20% emulsion templated titania coating at 5000×, c) nontemplated titania coating at 20,000×, and d) 20% emulsion templated titania coating at 20,000×.
S. Phadke et al. / Materials Letters 63 (2009) 2619–2621
2621
Fig. 3. Cross-sectional micrograph of 20% emulsion templated titania coating: a) microstructure 5000×, and b) microstructure at 20,000×.
around 45–50 m2/g surface area. Both the plots in Fig. 1 show a distinct peak at approximately 30 nm corresponding to the mesoporosity in the films arising from the nanocrystalline structure of the titania particles. The important difference, however, is the presence of two extra peaks at around 100 to 500 nm in the templated sample, which arise from the additional porosity created by the oil droplets. The presence of two peaks is thought to be a result of partial coalescence of oil droplets during the coating drying stage and is consistent with the view provided by SEM. It is interesting to note that mercury porosimetry relies on having the pore structure be interconnected to allow the intrusion of Hg into the pore structure to allow its quantification. So the fact that extra peaks are seen at the larger pore sizes is a direct indication that the emulsion templating process creates a fully interconnected pore network in the bulk of the final coating. This is also confirmed with cross-sectional SEM. Fig. 2 shows the microstructure of the 20% emulsion templated coating (b and d) in comparison to the non-templated coating (a and c). In the lower magnification micrographs (a and b), the macropores created by removal of the oil droplets during heat treatment can be easily seen. In the higher magnification micrographs (c and d), the dual porous microstructure is more obvious. Fig. 3 shows the crosssectional micrograph of the 20% emulsion templated coating. From this micrograph it is obvious that the added pores are uniformly present in the bulk of the coating. 4. Conclusion Oil in water emulsions were used to incorporate meso- and macroporosity into titania coatings, useful as photoanodes for dye sensitized solar cells. The pore size and pore size distribution can be easily controlled by this method by changing the formulation of the emulsion. The interpenetrating pore structures would allow easier penetration of dye and electrolyte into the titania film which can improve the photocurrent and photovoltage parameters. Microstructures, studied using mercury porosimetry and electron microscopy, showed the presence of interconnected pores in the range of 30– 1000 nm. Future experiments involve comparison of the solar cell performance of the emulsion templated coatings with respect to the non-templated coatings. Acknowledgement We would like to thank the NSF Ceramic and Composite Materials Center for supporting this project.
References [1] Anandan S. Recent improvements and arising challenges in dye-sensitized solar cells. Sol Energy Mater Sol Cells 2007;91:843–6. [2] Gratzel M. Progress in photovoltaics: research and applications. Prog Photovolt Res Appl 2000:171–85. [3] O'Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991;353:737–40. [4] O'Rourke S, Kim P, Polavarapu H. Solar photovoltaics: technology and economics: thin films and crystalline silicon, in Deutsche Bank — global market research report; 2007. [5] Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J, et al. Solid-state dyesensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 1998;395:583–5. [6] Jiu J, Wang F, Sakamoto M, Takao J, Adachi M. Preparation of nanocrystalline TiO2 with mixed template and its application for dye-sensitized solar cells. J Electrochem Soc 2004;151:A1653–8. [7] Kopidakis N, Benkstein KD, Lagemaat J, Frank AJ. Transport-limited recombination of photocarriers in dye-sensitized nanocrystalline TiO2 solar cells. J Phys Chem B 2003;107:11307–15. [8] Qi L, Birnie III DP. Templated titania films with meso- and macroporosities. Mater Lett 2006;61:2191–4. [9] Qi L, Sorge JD, Birnie III DP. Dye sensitized solar cells based on TiO2 coatings with dual size-scale porosity to appear. J Am Ceram Soc 2009. [10] Abdelsalam ME, Bartlett PN, Baumberg JJ, Coyle S. Preparation of arrays of isolated spherical cavities by self assembly of polystyrene spheres on self assembled prepatterned macroporous films. Adv Mater 2004;16:90–3. [11] Holland BT, Blanford CF, Stein A. Synthesis of highly ordered three-dimensional mineral honeycombs with macropores. Science 1998;281:538–40. [12] Jiang P, Cizeron J, Bertone JF, Colvin VL. Preparation of macroporous metal films from colloidal crystals. J Am Chem Soc 1999;121:7957–8. [13] Jiu J, Wang F, Sakamoto M, Takao J, Adachi M. Performance of dye-sensitized solar cell based on nanocrystals TiO2 film prepared with mixed template method. Sol Energy Mater Sol Cells 2005;87:77–86. [14] Wijnhoven JE, Vos WL. Preparation of photonic crystals made of air spheres in titania. Science 1998;281:802–4. [15] Zhong Z, Yin Y, Gates B, Xia Y. Preparation of mesoscale hollow spheres of TiO2 and SnO2 by templating against crystalline arrays of polystyrene beads. Adv Mater 2000;12:206–9. [16] Zhou Z, Zhao XS. Opal and inverse opal fabricated with a flow-controlled vertical deposition method. Langmuir 2005;21:4717–23. [17] Zukalová M, Zukal A, Kavan L, Nazeeruddin MK, Liska P, Gratzel M. Organized mesoporous TiO2 films exhibiting greatly enhanced performance in dye-sensitized solar cells. Nano Lett 2005;5:1789–92. [18] Schacht S, Huo Q, Voigt-Martin IG, Stucky GD, Schüth F. Oil–water interface templating of mesoporous macroscale structures. Science 1996;273:768–71. [19] Imhof A, Pine DJ. Ordered macroporous materials by emulsion templating. Nature 1997;389:948–51. [20] Imhof A, Pine DJ. Uniform macroporous ceramics and plastics by emulsion templating. Chem Eng Technol 1998;21:682–5. [21] Imhof A, Pine DJ. Preparation of titania foams. Adv Mater 1999;11:311–4. [22] Li Y, Hagen J, Scha¤rath W, Otschik P, Haarer D. Titanium dioxide films for photovoltaic cells derived from a sol–gel process. Sol Energy Mater Sol Cells 1999;56:167–74.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Emulsion templating to obtain dual-size-scale mesoporous titania coatings Sarika Phadke ⁎, Jay Ho, Dunbar P. Birnie III Materials Science and Engineering, Rutgers University, Piscataway, New Jersey-08854, United States
a r t i c l e
i n f o
Article history: Received 21 August 2009 Accepted 7 September 2009 Available online 13 September 2009 Keywords: Emulsion templating Porosity Microstructure Solar energy materials
a b s t r a c t Meso- and macroporous TiO2 coatings have been fabricated using oil-in-water emulsions as the templating phase. Coatings with interpenetrating, polydisperse pore structures are obtained which are useful for applications such as dye sensitized solar cells (DSSC), where permeation of one or more reactants into the inner surface areas of the coating microstructure is critical. Very uniform, defect free coatings are obtained with commercially available TiO2 nanopowder, with pores in the range of 30–1000 nm. Characterization of the coatings was performed using mercury porosimetry and electron microscopy. © 2009 Published by Elsevier B.V.
1. Introduction Dye sensitized solar cells using nanocrystalline TiO2 coatings as the photoanode, are technically and economically strong competitors to the existing photovoltaic technologies [1–4]. During the dye sensitized solar cell operation, the dye molecules, adsorbed on the titania coating, get excited and inject electrons into the conduction band of the TiO2 particles. The nanometer sized, sintered TiO2 particles allow electronic conduction towards the transparent conductive oxide (TCO) and the dye molecule recovers back to its initial ground state by oxidizing the electrolyte. The TiO2 photoanode is supposed to perform the following four major functions in the solar cell: (1) provide surface for the dye adsorption, (2) accept electrons from the excited dye, (3) conduct electrons to the TCO, and (4) allow for regeneration of the dye by giving access to the electrolyte through the porous structure. In order to perform the first and the last functions as stated above, the titania film should possess meso- (2–50 nm sized pores) and macro- (>50 nm pores) porosity with interconnecting pore channels [5–9]. Several methods have been tried to obtain a porous structure of TiO2 nanoparticles including combination of sol–gel and templating techniques [10–17]. Jiu et al. [13] used copolymer F-127 (poly (ethylene oxide)106–poly(propylene oxide)70–poly(ethylene oxide)106) and surfactant CTAB (cetyltrimethylammonium bromide) to obtain templated growth of porous TiO2 films. A similar structure was obtained by Zukalová et al. [17] using Pluronic P-123. Although a high surface area was achieved in these works, the pore diameters obtained were quite small in the range of 4–8 nm which would affect the diffusion of redox species and penetration of organic hole conductor adversely. In the earlier work of our research group, we demonstrated a simple mixing–casting
⁎ Corresponding author. E-mail address: [email protected] (S. Phadke). 0167-577X/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.matlet.2009.09.021
method to obtain templated TiO2 coating with meso- and macroporosity and improved film quality [8]. In that work we had used monosized polystyrene particles to template the interconnected pore structures. Interestingly, we found that our best solar cell efficiency was achieved when the templates had created a percolating interconnected – but still disordered – network of large-size pore channels [9]. That observation paved the way for the present work where a random emulsion was used for templating. In this paper we report on this alternative, cost effective templating method using oil-in-water emulsions as the templating phase. In earlier work oil/water interface templating has been used to obtain acid-prepared mesostructures of silica on two different length scales [18]. Imhof and Pine [19–21] demonstrated use of emulsion templating in the sol–gel process to produce macroporous silica. Since most of the metal alkoxides, including titanium alkoxide, lead to immediate precipitation in the presence of an aqueous medium, Imhof and Pine used non-aqueous emulsions to obtain porous titania. In the present work, we have employed simple aqueous emulsions to incorporate meso- and macroporosity into titania coatings, starting with pre-synthesized titania dispersion. This method provides a much less expensive and simpler way to obtain dual porosity TiO2 coatings with uniform, crack free film quality.
Table 1 Particle size measurement of emulsions with different oil concentrations. Oil concentration (%)
Mean diameter — volume averaged (nm)
10 20 30 40
107 165 3900 5650
2620
S. Phadke et al. / Materials Letters 63 (2009) 2619–2621
were dried in air for 15 min and then annealed at 500 °C for 30 min with a ramping rate of 2°/min. Characterization: Brookhaven Instrument's ZetaPALS 90Plus particle size analyzer was used to measure the particle size distribution of the oil droplets created in the oil-in-water emulsions. The pore structure was analyzed using Micromeritics AutoPore 9400 mercury porosimeter and the appearance of the microstructure was studied using a Zeiss-DSM 982 scanning electron microscope. 3. Results and discussion
Fig. 1. Mercury porosimetry plots for non-templated and 20% emulsion templated coatings. Templated plot shows extra porosity sizes created by oil droplets during drying.
2. Experimental Synthesis: A stable oil-in-water emulsion was prepared using paraffin oil and Tween 80/Span 80 emulsifier blend obtained from Sigma-Aldrich. Titanium dioxide nanoparticles (Aeroxide-P25) were obtained from Evonik/Degussa. Aeroxide P25 contains 70% anatase and 30% rutile titania. Upon heat treatment above 450 °C mainly anatase phase exists [22].Oil-in-water emulsions were prepared with effective HLB (hydrophile–lypophile balance) of the emulsifier blend set to 11. The P25 titania nanoparticles were then dispersed in the emulsion using Hamilton Beach Scovill 936 homogenizer. 10 to 20 micron thick coatings were doctor bladed on fluorinated tin oxide (FTO) coated glass substrates using scotch tape to control the thickness. The coatings
Table 1 gives volume averaged diameters of the oil droplets as a function of composition for four mixing ratios: 10, 20, 30 and 40%. It was observed that the 30% and 40% emulsions started to sediment after 15–20 min, which was a result of much bigger oil droplet size, indicating that these had not been stabilized well enough for further testing. Therefore emulsions with 20% oil were used for further experiments discussed below. Upon dispersing titania nanoparticles into the 20% oil-in-water emulsion, a thick paste was obtained. This paste was doctor bladed onto cleaned FTO coated glass substrates. The thickness of the coating was controlled by two pieces of scotch tape. The coating was allowed to air dry slowly and then heat treated to 500 °C for 30 min. Typically, the final coating thickness, measured by profilometry, was around 15–20 µm with the templated solution and around 10–15 µm with the nontemplated solution. Some coatings were gently scrapped off the substrate to gather enough material for accurate mercury porosimetry. Fig. 1 shows the comparison of the results obtained with the mercury porosimetry, for non-templated and 20% emulsion templated samples. The non-templated titania sample showed 53% porosity while the 20% emulsion templated sample showed 69% porosity. The BET specific surface area on the other hand did not differ much. Both the samples had
Fig. 2. Microstructure evaluation with scanning electron microscopy: a) non-templated titania coating at 5000×, b) 20% emulsion templated titania coating at 5000×, c) nontemplated titania coating at 20,000×, and d) 20% emulsion templated titania coating at 20,000×.
S. Phadke et al. / Materials Letters 63 (2009) 2619–2621
2621
Fig. 3. Cross-sectional micrograph of 20% emulsion templated titania coating: a) microstructure 5000×, and b) microstructure at 20,000×.
around 45–50 m2/g surface area. Both the plots in Fig. 1 show a distinct peak at approximately 30 nm corresponding to the mesoporosity in the films arising from the nanocrystalline structure of the titania particles. The important difference, however, is the presence of two extra peaks at around 100 to 500 nm in the templated sample, which arise from the additional porosity created by the oil droplets. The presence of two peaks is thought to be a result of partial coalescence of oil droplets during the coating drying stage and is consistent with the view provided by SEM. It is interesting to note that mercury porosimetry relies on having the pore structure be interconnected to allow the intrusion of Hg into the pore structure to allow its quantification. So the fact that extra peaks are seen at the larger pore sizes is a direct indication that the emulsion templating process creates a fully interconnected pore network in the bulk of the final coating. This is also confirmed with cross-sectional SEM. Fig. 2 shows the microstructure of the 20% emulsion templated coating (b and d) in comparison to the non-templated coating (a and c). In the lower magnification micrographs (a and b), the macropores created by removal of the oil droplets during heat treatment can be easily seen. In the higher magnification micrographs (c and d), the dual porous microstructure is more obvious. Fig. 3 shows the crosssectional micrograph of the 20% emulsion templated coating. From this micrograph it is obvious that the added pores are uniformly present in the bulk of the coating. 4. Conclusion Oil in water emulsions were used to incorporate meso- and macroporosity into titania coatings, useful as photoanodes for dye sensitized solar cells. The pore size and pore size distribution can be easily controlled by this method by changing the formulation of the emulsion. The interpenetrating pore structures would allow easier penetration of dye and electrolyte into the titania film which can improve the photocurrent and photovoltage parameters. Microstructures, studied using mercury porosimetry and electron microscopy, showed the presence of interconnected pores in the range of 30– 1000 nm. Future experiments involve comparison of the solar cell performance of the emulsion templated coatings with respect to the non-templated coatings. Acknowledgement We would like to thank the NSF Ceramic and Composite Materials Center for supporting this project.
References [1] Anandan S. Recent improvements and arising challenges in dye-sensitized solar cells. Sol Energy Mater Sol Cells 2007;91:843–6. [2] Gratzel M. Progress in photovoltaics: research and applications. Prog Photovolt Res Appl 2000:171–85. [3] O'Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991;353:737–40. [4] O'Rourke S, Kim P, Polavarapu H. Solar photovoltaics: technology and economics: thin films and crystalline silicon, in Deutsche Bank — global market research report; 2007. [5] Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J, et al. Solid-state dyesensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 1998;395:583–5. [6] Jiu J, Wang F, Sakamoto M, Takao J, Adachi M. Preparation of nanocrystalline TiO2 with mixed template and its application for dye-sensitized solar cells. J Electrochem Soc 2004;151:A1653–8. [7] Kopidakis N, Benkstein KD, Lagemaat J, Frank AJ. Transport-limited recombination of photocarriers in dye-sensitized nanocrystalline TiO2 solar cells. J Phys Chem B 2003;107:11307–15. [8] Qi L, Birnie III DP. Templated titania films with meso- and macroporosities. Mater Lett 2006;61:2191–4. [9] Qi L, Sorge JD, Birnie III DP. Dye sensitized solar cells based on TiO2 coatings with dual size-scale porosity to appear. J Am Ceram Soc 2009. [10] Abdelsalam ME, Bartlett PN, Baumberg JJ, Coyle S. Preparation of arrays of isolated spherical cavities by self assembly of polystyrene spheres on self assembled prepatterned macroporous films. Adv Mater 2004;16:90–3. [11] Holland BT, Blanford CF, Stein A. Synthesis of highly ordered three-dimensional mineral honeycombs with macropores. Science 1998;281:538–40. [12] Jiang P, Cizeron J, Bertone JF, Colvin VL. Preparation of macroporous metal films from colloidal crystals. J Am Chem Soc 1999;121:7957–8. [13] Jiu J, Wang F, Sakamoto M, Takao J, Adachi M. Performance of dye-sensitized solar cell based on nanocrystals TiO2 film prepared with mixed template method. Sol Energy Mater Sol Cells 2005;87:77–86. [14] Wijnhoven JE, Vos WL. Preparation of photonic crystals made of air spheres in titania. Science 1998;281:802–4. [15] Zhong Z, Yin Y, Gates B, Xia Y. Preparation of mesoscale hollow spheres of TiO2 and SnO2 by templating against crystalline arrays of polystyrene beads. Adv Mater 2000;12:206–9. [16] Zhou Z, Zhao XS. Opal and inverse opal fabricated with a flow-controlled vertical deposition method. Langmuir 2005;21:4717–23. [17] Zukalová M, Zukal A, Kavan L, Nazeeruddin MK, Liska P, Gratzel M. Organized mesoporous TiO2 films exhibiting greatly enhanced performance in dye-sensitized solar cells. Nano Lett 2005;5:1789–92. [18] Schacht S, Huo Q, Voigt-Martin IG, Stucky GD, Schüth F. Oil–water interface templating of mesoporous macroscale structures. Science 1996;273:768–71. [19] Imhof A, Pine DJ. Ordered macroporous materials by emulsion templating. Nature 1997;389:948–51. [20] Imhof A, Pine DJ. Uniform macroporous ceramics and plastics by emulsion templating. Chem Eng Technol 1998;21:682–5. [21] Imhof A, Pine DJ. Preparation of titania foams. Adv Mater 1999;11:311–4. [22] Li Y, Hagen J, Scha¤rath W, Otschik P, Haarer D. Titanium dioxide films for photovoltaic cells derived from a sol–gel process. Sol Energy Mater Sol Cells 1999;56:167–74.