Assembly of Highly Ordered Three-Dimensional Porous Structure with Nanocrystalline TiO 2 Semiconductors

Transcription

1 Chem. Mater. 2002, 14, Assembly of Highly Ordered Three-Dimensional Porous Structure with Nanocrystalline TiO 2 Semiconductors Q.-B. Meng,*,, C.-H. Fu, Y. Einaga, Z.-Z. Gu, A. Fujishima,, and O. Sato*, Special Research Laboratory for Optical Science, Kanagawa Academy of Science and Technology, KSP Building East 412, Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa, Japan, State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, Beijing , China, and Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1, Hongo Bunkyo-ku, Tokyo , Japan Received February 22, Revised Manuscript Received October 11, 2001 In this paper, we report on the assembly of a highly ordered three-dimensional porous structure with nanosized crystalline TiO 2 particles by a cooperative assembly method in which the fabrication of the template and the infiltration of the voids of the template are carried out at the same time and the related experimental parameters for the assembly, including temperature, humidity, and concentrations and concentration ratio of the colloid mixture. SEM (scanning electron microscope) images and transmission spectra of these samples demonstrate that these films have a highly ordered three-dimensional structure. The Bragg law was used to calculate the diameter of the spheres of air in the porous TiO 2 structure. A good agreement between the calculated results for the diameter of the spheres of air and those measured by SEM further confirms the high quality of the films fabricated using this simple method. Additionally, based on these experimental results, a detailed mechanism of the simple method is also discussed. Introduction Porous nanocrystalline TiO 2 semiconductors have recently attracted much attention because of their various applications in electronic, electrochemical, and photocatalytic systems, including photoelectrochemical solar cells, 1-3 electrocatalysts, 4-6 sensors, 7,8 and highperformance photocatlysts. 9 In particular, highly ordered three-dimensional (3-D) porous TiO 2 structures with lattice spacings on the order of wavelengths of light can be used as PBG (photonic band gap) materials because TiO 2 crystals have a large refractive index and are transparent in the visible light region. 10,11 Moreover, catalysis, large-molecule separation processes, and other * Corresponding authors. qbmeng@aphy.iphy.ac.cn and mengqingbo@hotmail.com. sato@fchem.chem.t.u-tokyo.ac.jp. Kanagawa Academy of Science and Technology. Chinese Academy of Sciences. The University of Tokyo. (1) Barbe, C. J.; Arendse, F.; Comte, P. Jirousek, M.; Lenzmann, F.; Shklover, V.; Gratzel, M. J. Am. Ceram. Soc. 1997, 80, (2) ORegan, B.; Schwartz, D. T.; Zakeeruddin, S. M.; Gratzel, M. Adv. Mater. 2000, 12, (3) Park, N. G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, (4) Hoyer, P. Langmuir 1996, 14, (5) Moriguchi, I.; Maeda, H.; Teraoka, Y.; Kagawa, S. Chem. Mater. 1997, 9, (6) Lakshmi, B. B.; Dorhourt, P. K.; Martin, C. R. Chem. Mater. 1995, 9, 857. (7) Hoyer, P.; Masuda, H. J. Mater. Sci. Lett. 1996, 15, (8) Fujishima, A.; Rao, N. T.; Tryk, D. A. Electrochim. Acta 2000, 45, (9) Gopidas, K. R.; Bohorques, M.; Kamat, P. V. J. Phys. Chem. 1990, 94, (10) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (11) Rengarajan, R.; Jiang, P.; Colvin, V.; Mittleman, D. Appl. Phys. Lett. 2000, 77, applications would also benefit from more uniform porous supports that provide optimal flow and improve efficiencies. The most common method used for assembling highly ordered porous structures is the template method A major advantage of this method is that the dimensions of the pores are set by the size of the ordered template beads and the pore size can be varied easily. In the generalized template method, 3-D templates must first be fabricated, and then the voids of the template must be infiltrated. Hereafter, we call this generalized method the two-step method. It is wellknown that three challenging problems, namely, (1) preparation of a high-quality template, (2) complete infiltration of the voids of the template without damage of the template itself when filled, and (3) minimization of shrinkage after removal of the template, directly influence the quality of the porous structure fabricated by the two-step method. The biggest barrier for fabricating highly ordered porous structure using the twostep method is to resolve the three challenging problems at the same time. Furthermore, one important limitation of the general two-step template method is that the quality of the porous structure is strictly limited by the quality of the template itself. (12) Yin, J. S.; Wang, Z. L. Adv. Mater. 1999, 11, 469. (13) Gates, B.; Yin, Y.; Xia, Y. Chem. Mater. 1999, 11, (14) Vlasov, Y. A.; Yao, N.; Norris, D. J. Adv. Mater. 1999, 11, 165. (15) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, (16) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, (17) Velev, O. D.; Kaler E. Adv. Mater. 2000, 12, /cm CCC: $ American Chemical Society Published on Web 12/01/2001

2 84 Chem. Mater., Vol. 14, No. 1, 2002 Meng et al. Figure 1. Schematic of the assembly method. Sucking capillary pressure is directly used to drive the nanosized particles to assemble themselves in the ordered template while the template is being fabricated using the generalized vertical deposition method. Based on the advantages of several fabrication methods, we have, more recently, developed a simple method to solve all three problems discussed above at the same time. 22 By utilizing the local sucking capillary pressure, 23 ultrafine particles can be used directly to assemble themselves in the voids of template while the template is being assembled. Highly ordered SiO 2 porous structures with large areas and uniform orientations have been fabricated successfully using this method. 22 In this paper, we report on the assembly of highly ordered TiO 2 porous structure using this simple method and provide details on experimental parameters, such as temperature, humidity, and concentrations and concentration ratio of the colloid mixture. The detailed mechanism of the simple method is also discussed in this paper. We believe that this work will stimulate the wide application of highly ordered TiO 2 porous structure films fabricated using nanosized TiO 2 semiconductor particles. Mechanism of the Simple Method Our basic idea is to infill ultrafine particles (around 10 nm) into the voids of an ordered template directly by means of capillary forces. Figure 1 shows the schematic of the simple fabrication method used. The substrate is dipped into a slurry that contains polystyrene spheres (several hundred nanometers) and the ultrafine particles. In contrast to previously reported methods, we employ a vertical deposition technique, 21 not only because the technique itself can fabricate high-quality opal, but also because this fabrication method can help realize our overall strategy. Because of the small size of the TiO 2 particles (around 10 nanometers) compared with that of the polystyrene spheres (several hundred nanometers), the TiO 2 particles are immersed in the liquid layer when the ordered (18) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature 1999, 401, 548. (19) Subramania, G.; Constant, K.; Biswas, R.; Sigalas, M. M.; Ho, K. M. Appl. Phys. Lett. 1999, 74, (20) Subramania, G.; Manoharan, V. N.; Thorne, J. D.; Pine, D. J. Adv. Mater. 1999, 11, (21) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, (22) Meng, Q. B.; Gu, Z. Z.; Sato, O.; Fujishima, A. Appl. Phys. Lett. 2000, 77, (23) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, template is assembling. With the evaporation of liquid fluxes through the voids of the template, the nanosized TiO 2 particles can easily move and fill the voids of the template as a result of a convective water flux that carries along the particles toward the voids. A detail discussion about this kind motion is provided by Denkov et al. 23 The distinctive feature of this method is that the fabrication of the template and the infiltration of the ultrafine particles are carried out at the same time. Using this strategy, it is favorable for the ultrafine particles to assemble themselves in the voids of the template, as they need only move a short distance in the voids before assembling. Therefore, the voids can be completely filled by the ultrafine particles. Furthermore, in this simple fabrication process, the ultrafine particles can immediately mend several kinds of defects in the template when the template is assembling. It is well-known that defects in fabricated opal are induced by the random distribution of the diameters of the spheres of the template. For example, when a larger template sphere is introduced into the opal, sphere vacancies must be created around it, and when a smaller template sphere is introduced into the opal, a large void is formed around it. These defects can easily be mended by the ultrafine particles used in this simple fabrication process. Therefore, we call this simple method the cooperative assembly method. It is the immediate mending in the cooperative assembly process that greatly improves the fabrication of the template itself. In addition, this mending can improve the mechanical stability of the composite film. It is thought that the formation of cracks in opals is initiated at these defects. The complete filling of the defects might therefore lead to a reduction in the generation of cracks in fabricated opal, thus allowing for potential improvement in the quality of the porous structures. Because of this important advantage, the quality of the porous materials produced using this method is generally better than that of the template itself, which is fabricated by a general vertical deposition method under the same experimental conditions, except for the absence of the ultrafine particles in the solution. Another advantage of this method is that it is very simple and can be widely used. Experimental Section Materials and Substrates. The monodisperse polystyrene particles used were obtained from Duke Scientific Corporation. Titania dispersions were purchased from Catalysts and Chemical Industrial Co. Ltd. The mean size of the titania particles used was about 13 nm. Ultrapure water (20.0 MΩ cm -1 ) was used directly from a Milli-Q water system. The substrates used in our experiments were glass slides, ITO glass, and quartz glass. Glass vials (10 ml, Iuchi) were used as the experimental cells. Instrumentation. Scanning electron microscopy experiments were carried out on a JSM-5400 scanning microscope. Transmission spectra were obtained using a Shimadzu UV- 3100PC spectrometer. An advanced AE-215 constant-temperature and -humidity chamber from the Toyo Seisakusho Co. Ltd. was used to control the temperature and humidity in these experiments. Fabrication of the Highly Ordered Porous Structure with Ultrafine TiO 2 Semiconductor Particles. Prior to use, all substrates and glass vials were soaked in a chromicsulfuric acid cleaning solution overnight, rinsed thoroughly with ultrapure water, and dried in a stream of nitrogen. The optimized concentrations of polystyrene and titania were about % by volume. The concentration can be varied from

3 Nanocrystalline TiO 2 Semiconductors Chem. Mater., Vol. 14, No. 1, Figure 2. Scanning electron micrographs at different magnifications illustrating the [111]-oriented regions of the TiO 2 porous structure films on ITO glass. All of the scanning electron micrographs in this work were taken with a JSM scanning microscope to 0.5%, and the concentration ratio is about 1:1 for our experimental conditions. After the full ultrasonic dispersion of the mixture slurry of the polystyrene and titania colloids, a clean substrate was then placed vertically into the mixture slurry in a clean glass vial. We found that the concentrations of the colloidal mixture, the concentration ratio between the individual colloids, and monodispersity of the mixture colloid, rather than the coating substrate, were the key parameters in controlling the film deposition. Subsequently, the glass vial was covered with a plastic film punched through with a few small holes to keep out any external airflow. Then, the glass vial was placed into a constant-temperature, constant-humidity chamber. The optimized temperature and humidity are 50 C and 30%, respectively. The area of film fabricated under these experimental conditions can reach about 0.5 cm 2 in 24 h. The number of layer (or total thickness) of the films can be controlled by the concentration of the slurry mixture or repeat deposition times under the same experimental conditions. After removal of the polystyrene spheres in the composite opal by calcination (the composite opal was heated slowly to 450 C in 7 h), a highly ordered three-dimensional TiO 2 porous structure was obtained. Results and Discussion Figure 2a and 2b shows the SEM images of TiO 2 porous structures at different magnifications on ITO glass. They show that a highly ordered hexagonal array is produced. The hexagonal orientation indicates the (111) plane of the fcc lattice. In Figure 2b, the holes Figure 3. Scanning electron micrographs at different magnifications illustrating [100]-orientation regions of the TiO 2 porous structure films on ITO glass. connecting the pores are clearly visible. In particular, a triangular pattern below each hole in the first layer can be clearly observed. This is because, along the [111] direction, each sphere of air rests on three neighboring spheres below. The observation of a regular triangular pattern over a large area by SEM strongly confirms the perfect three-dimensional ordering of the structure. Figure 2b shows that spherical vacancy defects in the template have been completely filled with nanosized TiO 2 semiconductor particles and that one small hole (indicated by an arrow) is surrounded by more TiO 2 semiconductor particles. These experimental results directly demonstrate the existence of cooperative assembly effects in the simple fabrication method. Although the same kind of phenomena can also been found in the samples fabricated by the two-step method, which fabricate the template first and then fill the voids of template, they are formed by different processes, and the effects on the final porous structure are different. It is the immediate mending of these defects by this simple cooperative assembly process that can greatly improve the fabrication of the template itself and, therefore, the quality of the final porous structure. Figure 3a and 3b shows SEM images of the [100]- direction zone of the sample at different magnifications. From Figure 3a, the low-magnification SEM image, we can see a highly ordered square array over a larger area. From Figure 3b, the high-magnification SEM image of

4 86 Chem. Mater., Vol. 14, No. 1, 2002 Meng et al. Figure 4. Typical scanning electron micrograph of the coexistence zone of the [111] and [100] orientations of the TiO 2 porous films on ITO glass. Figure 5. Scanning electron micrograph of the special zone of the TiO 2 porous films on ITO glass. the sample, we can clearly see the holes connecting the pores and, inparticular, a regular cross pattern below each hole of the first layer. This is because, along the [100] direction, each sphere of air rests on the four neighboring spheres below. The observation of a regular cross pattern strongly confirms the perfect threedimensional structure in this zone of the film. Figure 4 shows the [111]- and [100]-direction coexistence zone. At the boundaries between the [111]- and [100]-direction zones, we can clearly see that the zones are very well connected together. There are no transition zones between them. Furthermore, in the [111]-direction zone, we can clearly see a regular triangular pattern below each hole of the first layer, and in the [100]- direction zone, we can also clearly see a regular cross pattern below each hole of the first layer. These regular patterns directly confirm the three-dimensional structure of the different orientation zones. Figure 5 shows one special pattern in some zones of the sample fabricated by this method. In the [100]- direction zone, a regular [111]-direction zone is relayed. The two zones form a beautiful pattern over a larger area. By carefully examining the boundary between the two kinds of zones, we can also see a perfect connection between them. From the different regular patterns below the holes of the first layer in different zones, we Figure 6. Scanning electron micrograph showing a side view of the TiO 2 porous structures. can confirm the three-dimensional structures of the different zones. Figure 6 shows a side view of the porous structure. The cross section shows that the closely packed structure of the spheres of air extends uniformly over 10 layers produced during a single deposition. This side view SEM image also demonstrates the 3-D structure of the film fabricated by this simple method. By carefully examining most TiO 2 samples fabricated using this method, we find that the [111]-direction zones comprise 35% of the total area and that such zones can extend to several square millimeters in size. The [ 100]- direction zones are also about 35% of the total area, and such zones can extend to nearly 10 mm 2 in area. The special pattern zones are about 30% of the whole area, and such zones can extend to several square millimeters in size. Moreover, within one orientation zone or between zones, the whole sample is planar, and the thickness is uniform. By comparing the structure of porous TiO 2 with that of SiO 2, 22 both of which were fabricated by using the same method, we find that the domain sizes of SiO 2 are generally larger than those of TiO 2. The dominant direction of the SiO 2 porous structures is [111] (the [111]-direction regions comprise more than 75% of the whole sample area). In contrast, the dominant direction of TiO 2 porous structure is the [100] direction (the total percent of the [100] direction and special pattern area is more than 65% of the whole area of the TiO 2 sample). Although we cannot provide a detailed explanation as to why the dominant directions are different in the SiO 2 and TiO 2 porous structures at present, we believe that the main reason might be the large difference between the sizes of the ultrafine particles used. The size of the SiO 2 particles (around 7 nm) is much smaller than that of the TiO 2 particles (over 12 nm). Figure 7a shows transmission spectra of TiO 2 films with different pore sizes fabricated by this simple method. The first-order diffraction peaks can be clearly seen in the transmittance spectra. It should be noted here that the spot size of the light beam in the transmission experiments is about 12 2mm 2. The results further confirm that the porous structures are highly ordered over a large area. The diffraction peak positions are at 576, 698, 735, and 986 nm for ordered pore structures of different pore sizes. Figure 7b shows

5 Nanocrystalline TiO 2 Semiconductors Chem. Mater., Vol. 14, No. 1, Table 1. Calculated and SEM-Measured Values of the Pore Size of the TiO 2 Porous Structures Fabricated with Polystyrene Spheres of Different Diameters n eff a closest-packed TiO 2 particles whole porous structure peak position (nm) pore diameter (nm) calculated by Bragg law measured by SEM a n eff means the effective refractive index. light and the surface normal of the sample, and d is the distance between parallel lattice planes. In this calculation d ) d 100 ) (2.0) 1/2 R (2) Figure 7. (a) Transmission spectrum of TiO 2 porous structures with different pore sizes. (b) Photonic band gap position as a function of the pore size. The straight line is a linear fit to the peak wavelengths. Very good scaling with the pore size is observed. the linear relationship of the PBG position to the pore size. This indicates the intrinsic feature of the porous structure. The results demonstrate that highly ordered TiO 2 films with different pore sizes have been successfully fabricated using this method. Table 1 lists the effective refractive indexes of the closest-packed films of ultrafine TiO 2 particles and of the ordered TiO 2 porous structures, the calculated pore diameters, and the diameters measured by SEM. In the calculation of the pore diameters, we use the Bragg equation λ max ) 2dn a sin θ (1) where n a is the average refractive index of the photonic crystalline assembly, θ is the angle between the incident where R is the radius of spheres of air in the porous structures, because the dominant direction of TiO 2 porous structure fabricated under our experimental conditions is the [100] direction. The value of the refractive index for the TiO 2 particles used in this calculation is 2.46, because the TiO 2 ultrafine particle sample contains about 96% TiO 2 and 4% SiO 2 (the average refractive index of TiO 2 sample used in our experiment is 2.46 ) % %). We also assumed that the TiO 2 particles are in the closestpacked state (occupying 74 vol % of the total voids) in the voids of the template in this cooperative assembly process. A comparison of the values of the diameter of the spheres of air with those measured by SEM indicates good agreement between them. This further demonstrates that a highly ordered porous structure exists in three dimensions and confirms the closestpacked state of the ultrafine particles in the voids of the template. In other words, the ultrafine particles can completely fill the voids of the template in the cooperative assembly process. Comparing the pore diameter with that of the polystyrene measured by SEM, the shrinkage is between 4.2 and 6.8% for all samples. These values are slightly larger than those (shrinkage is between 3.2 and 4.6%) of SiO 2 porous films. 22 The main reason for this difference is the larger size of the TiO 2 used. Conclusions Using a very simple method, we have successfully assembled large-area, highly ordered porous structures using nanosized TiO 2 semiconductor particles. At present, we cannot strictly control the orientation of the porous structure in the whole sample to obtain a single orientation. However, the areas exhibiting a single orientation zone (on the order of several square millimeters) are large enough for specialized applications in optical science. Furthermore, our experimental results demonstrate that the smaller the ultrafine particle size, the better the results obtained for controlling the orientation of the porous structure fabricated and the easier the fabrication of a larger single domain using this method. The theoretical calculations confirm the existence of the closest-packed state of the ultrafine particles in the voids of the template. This special structure combines the advantages of nanosized TiO 2 particles with the

6 88 Chem. Mater., Vol. 14, No. 1, 2002 Meng et al. special optical properties of the structure itself. We believe that this kind of highly ordered TiO 2 porous structure, which were fabricated using nanosized TiO 2 particles, can be widely applied in the areas of electrocatalysts, sensors, high-performance photocatalysts, photonic crystals, and so on. As an additional note, we would like emphasis here that a high-quality 3-D porous structure can easily be fabricated by using this method with semiconductor quantum dots (whose size is around 3 nm). CM