Synthesis of nanometer-sized hexagonal disk-shaped ZnO in formic acid using a hydrothermal method and its optical properties

This study focused on the stable synthesis of nanometer-sized hexagonal disk-like ZnO, which can be utilized as the working electrode in DSSCs (dye-sensitized solar cells). Nanometer-sized zinc oxides, ZnOs, were successfully synthesized by hydrothermal treatment at 150, 200, and 250 °C for 8 h, and their morphologies were controlled by using different pHs, pH = 2, 3, and 4, with the addition of formic acid. The TEM (transmission electron microscopy) results reveal that the as-prepared particles at pH = 3 are hexagonal disk-shaped and the crystallite sizes are 40 nm measured across the diagonal. However, the shapes were different at the other pHs; nanoneedles at pH = 2 and hexagonal columns at pH = 4. The patterns of the photoluminescence (PL) spectra of the ZnOs varied according to their shapes; two types of emitting bands were observed in the case of the hexagonal nanodisks and columns at around 386–415 nm (violet) and 540–567 nm (green), respectively.