Preparation of Spherical- and Bowl-Shaped Mesoporous TiO2 Clusters as Light Scatterers and Their Application to Solar Cell and Photocatalysis

Abstract

Light can be scattered and redirected in many directions. Particularly, when the size of the particles is comparable to the wavelength of the incident light, the Mies scattering happens. In the atmosphere, the clouds appear white, attributing to the Mies scattering effect, which can scatter all wavelength of the visible light by the various sized dusts. If we introduce the Mies scattering phenomenon into the TiO2 spheres, when the light collides with the TiO2 sphere with size comparable to its wavelength, the light scatters strongly along the forward direction. Therefore, the path length of the incident light between the neighbored spheres will be increased, leading to a promoted light utilization efficiency over the spheres.

With this background and the technological and scientific value of light scatterers as motivations, the preparation of TiO2 clusters as light scattering centers with specific characteristics is both desirable and technologically important. In this study, spherical- and bowl- shaped mesoporous TiO2 clusters as light scattering centers were developed using aqueous and nonaqueous sol-gel process, then applied to solar cell and photocatalysis. Size-controlled spherical mesoporous TiO2 clusters were successfully synthesized using titanium precursor (titanium (IV) isopropoxide) and various amphiphilic triblock copolymers as soft template. The triblock copolymer is used as structure-directing and pore-forming agent and the interaction between the titanium precursor and the triblock copolymer self-assemblies in solvent leads to the assembly of TiO2 nanoparticles into spherical aggregates with wormhole-like porous structures. The triblock copolymer micelle was composed of a core dominated by PO and a corona dominated by EO segments. The core of the micelles was believed to be free of water, while the swollen corona was hydrated. The aggregation behavior of the triblock copolymer under thermodynamic conditions may affect the TiO2 cluster size and morphology. Based on size-controlled synthesis, we prepared the anatase TiO2/zinc phthalocyanine (ZnPc) hybrids in various sizes (245－1188 nm) and examined the visible light photocatalytic activity of TiO2/ZnPc hybrids, in conjunction with their Mie light scattering ability. ZnPc molecules are well-incorporated and dispersed on the surface of the TiO2, instead of being aggregates. We confirmed the formation of spherical TiO2/ZnPc hybrids in various sizes (245, 548, 798, 1188 nm in diameter), the large specific surface areas (up to 223.76 m2/g), and the mesoporous structures. The mesoporous TiO2/ZnPc hybrids showed excellent photocatalytic efficiency (up to 89.93% after 90 min) toward the degradation of methylene blue under visible light irradiation, which is about 2 times of that of simple nanoparticular P25/ZnPc. Among the mesoporous TiO2/ZnPc hybrids, the hybrid with a size of 548 nm (P123-TiO¬2/ZnPc) exhibited the highest photocatalytic activity. The highest activity observed on P123-TiO2/ZnPc may be attributed to its efficient cascade Mie scattering effect under visible light irradiation. Photoelectrodes composed of mesoporous TiO2 spheres as scattering centers and TiO2 nanoparticles as the binder were fabricated and tested with the aim of improving the energy conversion efficiency of dye-sensitized solar cells (DSSCs). These electrodes composed of TiO2 spheres and nanoparticles enable the fabrication of high performance DSSCs, because of the light scattering of the TiO2 spheres and the dye-loading capacity of their high surface areas. The energy conversion efficiency of composite-type photoelectrode was found to be 7.66%, which is higher than that of nanocrystal electrode (4.50%). We prepared the spherical and bowl-shaped clusters of titania (TiO2) nanorods using an electrospray technique and examine the photovoltaic properties of the clusters, in conjunction with their light scattering ability. The nanorods with a diameter of about 3 nm and the length of about 25 nm formed clusters with different morphology depending on the vapor pressure of the solvent. The clusters possessed high specific surface areas of up to 113.57 m2/g and mesoporous structures. Dye-sensitized solar cells containing the clusters as a scattering layer displayed excellent photovoltaic performance. The cell with a scattering layer of the bowl-shaped TiO2 nanorod cluster exhibited the highest energy conversion efficiency of the cells of 9.13%. Such high efficiency was attributed to the high dye loading capacity and multiple light scattering ability of the bowl-shaped TiO2 nanorod cluster.