Abstract:

ZnO microstructures and nanostructures with controlled-morphology were synthesized by the hydrothermal method. All samples were prepared using precursors at different pH values and then annealed at 500 ℃ for 2 h. The samples were characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), and BET specific surface area measurement. All samples were confirmed by XRD to be wurtzite ZnO. As the pH value of the precursor increased, sheet-like ZnO disappeared and rod-like ZnO was produced. The major surfaces of sheet-like and rod-like ZnO were polar and nonpolar crystal faces, respectively. At pH 6.5, Cl^- was adsorbed onto the (002) polar face and inhibited the growth along the polar crystal face ({Zn²⁺}_{crystal surface}). A microporous sheet ZnO was formed by annealing the obtained sheet-like Zn₅(OH)₈Cl₂·H₂O. When OH^- was added into the precursor, Zn(OH)₄^2- was generated via coordination with Zn²⁺, which was adsorbed onto the (002) polar face and promoted growth along the polar crystal face. Rod-like ZnO was thus produced. The obtained ZnO could photocatalytically reduce CO₂ under illumination. Sheet-like ZnO exhibited better photocatalytic performance than rod-like ZnO. This may be because the polar crystal face shows better photocatalytic activity than the unpolar crystal face.

Key words: ZnO, pH value, Hydrothermal process, Morphology, Photoreduction of CO₂