Microwave-assisted hydrothermal synthesis of structure-controlled ZnO nanocrystals and their properties in dye-sensitized solar cells

Abstract

Microwave-assisted hydrothermal synthesis of ZnO nanocrystals has resulted in two distinctive structures: hexagonal bi-pyramidal (HBP) and nano-sheet (NS) shapes from Zn(CH3COO)2 solutions containing triethanol-amine or benzene tetracarboxylic acid as structure directing agents, respectively. Observation by field-emission scanning electron microscope, high resolution transmission electron microscope as well as analysis of selected area electron beam diffraction patterns revealed that the predominantly exposed facets of HBP and NS ZnO are (102) and (100) faces, occupying 84 and 76% of the total surface area, respectively. Adsorption of D149 and eosin Y photosensitizer dyes onto HBP and NS ZnO was studied in comparison with a randomly structured commercial ZnO nanocrystal (MZ). Density of dye packing was found to be in the order of HBP > MZ > NS, for which HBP was 2 to 5 times higher than NS. Consequently, dye-sensitized solar cells employing HBP ZnO exhibited the highest short circuit photocurrent, but also the lowest open circuit voltage. By contrast, the DSSCs employing NS ZnO exhibited the highest voltage among the three.