Effects of different alkali metal ions on morphology and photocatalysis properties of ZnO crystals via hydrothermal method

Abstract

With the precursor solution prepared by ZnSO₄·7H₂O and MOH (M = Li, Na or K), ZnO nanocrystals single doped with Li, Na, K and codoped with Na and K were prepared via a simple hydrothermal method. Their morphologies and structures were characterized by SEM, XRD, ICP, XPS and ultraviolet-visible spectrophotometer methods. It is found that the crystal morphology changes from the short bar shape to the long strip shape; Li⁺ is likely easier than Na⁺ and K⁺ to enter into the lattice to replace Zn²⁺ and inhibits the growth of positive polar plane while promotes the growth of negative one of ZnO in hydrothermal environment, leading to the morphology of short bars; K⁺ is easier than Li⁺ and Na⁺ to promote the two-dimensional nucleation and accelerate the axial growth of ZnO, leading to the morphology of long strips; Na⁺ and K⁺ have the obvious synergistic effect to facilitate the access to the lattice of ZnO. Experimental result of photocatalytic degradation of methylene blue shows that the ratio I of intensitiy of X-ray diffraction peak for (002) plane to that of (100) plane can reflect the relative area proportion of plane (002) with strong photocatalytic activity, and the photocatalytic effect may be better if the value of I is larger.

Keywords:

• Zinc oxide
• hydrothermal synthesis
• alkali metal ion doping
• crystal morphology
• growth mechanism
• photocatalytic property