Characterization and Formation Process of Highly Crystallized Single Crystalline TiO2 Nanorods for Dye-Sensitized Solar Cells
Highly crystallized titania nanorods were prepared by a hydrothermal method, and their formation processes were investigated by characterizing reaction products in a series of desired time intervals by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), and scanning electron microscopy (SEM). A complex electrode of dye-sensitized solar cells was then fabricated using the high-crystalline nanorods thus prepared. In the initial reaction stage (0.5—3.5 h), film-like products with amorphous or amorphous-like structure were observed. In the film with amorphous phase, very tiny two-dimensional anatase crystals were formed as nuclei, and the number and size of the nuclei increased with time. The nuclei were identified to have plane-like shape with the intensity ratio (004)/(200)—signifying growth in the c-axis— being almost zero. When the intensity ratio reached around 0.4, indicating slight growth in the c-axis, a drastic change in the shape and crystalline structure was observed to take place. This change was found to occur in a very short period (3.5-4 h), resulting from the phase transition from film with amorphous-like structure to titania nanorods with anatase crystalline structure. It was at 6 h or later that the HRTEM images showed titanium atoms aligning perfectly in titania anatase structure. This series of findings evidenced the formation of highly crystallized titania with various shapes and encouraged the fabrication of high-efficiency dye-sensitized solar cells; high electronic conductivity of the highly crystallized nanorods was found to contribute to achieving a conversion efficiency as high as 8.52 %.
Keywords: Titania nanorods, Formation process, Dye-sensitized solar cells, Highly crystallized, Hydrothermal method, Amorphous phase
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