Positron Annihilation Study of Porous ZnO Synthesized with Soft Template

摘要

Porous ZnO were synthesized with soft template method using zinc acetate Zn(CH3COO)_2-2H_20 as precursor and block copolymer F127 as the surfactant. Nitrogen adsorption-desorption measurements indicate that the ZnO sample contains large pores with mean diameter of about 30 nm. However, both small-angle X-ray diffraction and transmission electron microscope measurements indicate that the pore ordering is missing. Positron lifetime measurements reveal two long lifetime components in the porous ZnO. The longest lifetime τ_4 (75ns) corresponds to ortho-positronium (o-Ps) annihilation in large pores. The pore size estimated from τ_4 is about 10.6 nm. This is much smaller than that estimated from Nitrogen adsorption-desorption measurements. In addition, the intensity I_4, is only about 2.2%. This is probably due to the chemical quenching and/or inhibition of positronium formation induced by ZnO, which reduces o-Ps lifetime and intensity, and leads to under estimation of the pore size.