Microscopic and Isotope Tracer Study on the Growth of Spherical ZnO Particles in Water–Ethylene Glycol Solvent

Abstract

ZnO spherical powder with hierarchical structure was synthesized from zinc acetate anhydride and hexamethylenetetramine in a solvent of water and ethylene glycol (EG), and its time evolution was investigated by microscopic and isotope tracer studies. Initially, spherical ZnO particles made of nanocrystallites were precipitated. From the early growth stage, crystallites were radially aligned along the c-axis. With aging, the nanoparticles transformed into hexagonal prism-like particles with pyramidal tops, which showed clear radial alignment. Subsequently, the ZnO spheres became cracked and broke into wedge-shaped pieces of the pyramid-like particles. The microscopic studies suggested that the spherical particles grew through oriented attachment of small particles during the early growth stage, while Ostwald ripening through dissolution−recrystallization changed the component particles to a pyramidal shape in the later stage. In order to investigate the growth process, we attempted a novel isotope tracer study, where H 2 18 O was used for the solvent and the distribution of 18 O in the spherical particles was analyzed by a secondary ion mass spectrometer. The concentric 18 O distribution in the ZnO spherical powder aided the mechanism of continuous concentric-oriented attachment of nanoparticles to form spherical ZnO particles. The difference in the 18 O distribution before and after transformation into the pyramid shape was small, which implies that this change was a local reaction. The oxygen isotope tracer study in water−EG solvent was proven effective in determining the history of the particles and the development of their inner structures.

Nanocone Decorated ZnO Microspheres Exposing the (0001) Plane and Enhanced Photocatalytic Properties

“…Comparing the products in the 85%EG and 95%EG systems, in particular, noticing that the formation of selforientated one dimensional nanocone chains only occur in 85%EG, we can conclude that water molecules are crucial to the nucleation of ZnO and crystal growth. This has been supported by the iso tope tracer study by Saito et al using H 2 18 O in the solvent, [32] where 18 O transferred from water molecules to the ZnO crys tals. The higher concentration of water in the 85%EG system partially overcame the diffusion problem of water into the spherical particles and enhanced Ostwald ripening in the cores, leading to the formation of nanocones.…”

“…Similar mesostructures were also observed by Saito et al, who confirmed that the ZnO spherical particles were made from wedgeshape units radially aligned along the caxis and the entire particle is terminated by a single c(+)plane. [22,32] However, the formation mechanism was not discussed in detail.…”

“…This model agrees with the previous CBED study on ZnO nano cones, in which it was confirmed that the flat base of nano cones, prepared with the same method as that used in this work, terminated at the c(+)plane. [22,32] The termination plane on the base of the nanocones can be changed to c(-)plane, if the principal ligands are changed to positively charged mole cules. For example, mixing oleic acid and ethylenediamine to make an ionic solvent, the flat base of ZnO hexagonal micro pyramids were terminated by the c(-)plane.…”

Selective Homoepitaxial Growth of ZnO Layers on c(+)-Surface by Solvothermal Reaction in Water–Ethylene Glycol Solvent

“…The cross-sectional view shows that the particles of the top layer had a pyramidal shape with the base of the pyramids directed toward the surface as component particles of the spheres. 27 , 28 …”

“…We have previously reported the synthesis of spherical ZnO particles in a water–ethylene glycol (EG) solvent. 25 − 28 The spherical particles had hierarchical structures and were composed of pyramidal-shaped crystallites that were radially aligned along the c -axis. Transmission electron microscopy (TEM) studies revealed that the pyramid particles had a c (+)-plane base and {101̅1̅} six side surfaces.…”