Oxide semiconductors have unique properties in various applications such as in transparent electrodes,
smart windows, solar cells, and light-emitting diodes (LED). Especially, zinc oxide (ZnO) has been widely
studied owing to outstanding electrical and optical properties. In general, ZnO adapts to a hexagonal wurtzite
structure. ZnO has interesting properties such as a wide and direct bandgap of 3.37 eV and a large exciton
binding energy of 60 meV at room temperature. In particular, low dimensional ZnO nanostructures like nanowires,
nanotubes, nanorings, and nanowalls show exceptional properties due to quantum confinement effects.
Moreover, as low dimensional nanostructures have high surface-to-volume ratios, it is good for optoelectronic
devices and chemical sensors. Recently, ZnO-ZnS core-shell or heterostructures have been widely studied
because of enhanced or newly designed properties. As the band gap of ZnS is 3.7 eV, which is higher than
ZnO, the photoluminescence quantum yields can be improved. Moreover, light and gas sensing properties is
also enhanced.
One of the most facile methods to make ZnO-ZnS nanostructures is using thioacetamide (TAA) and
ZnO sacrificial templates in water. This method is not only simple but also versatile to make ZnS-ZnO coreshell
and ZnS hollow structures. For these reasons, this method has been widely used and studied for a long
time. However, most researches are concentrated on the synthesis of core-shell or hollow structures and little
is known about phase formation and structural transformation during ZnO sulfidation process. In addition, the
sulfidation process could be affected by the morphology of ZnO structures. ZnS has two basic structures such
as cubic sphalerite, stable at room temperature, and hexagonal wurtzite, high temperature form. In addition,
many polytype structures such as 8H, and 15R are observed and have similar interplanar spacing. Therefore,
the classification of crystalline phase is difficult. Neverth...