Epitaxial ZnO nanowires (NWs) were synthesized on sapphire (001) substrates using a hydrothermal process. The effects of the pH value of the precursor solution on the structural and optical properties of the resulting NWs was studied. The epitaxial relationship and the domain matching configuration between the sapphire (001) substrate and the as-grown ZnO NWs were determined using synchrotron X-ray diffraction measurements. The (002) plane of wurtzite ZnO NW grows in the surface normal direction parallel to the sapphire (001) direction. However, three types of in-plane domain matching configurations were observed, such as the on-position, 30°-rotated position, and ±8.5°-rotated position relative to the on-position, which might be attributed to inheriting the in-plane domain configuration of the ZnO seed layer.

Fe₃O₄ was prepared on the TiO₂-coated natural mica substrate. The natural mica has an average particle size of 22 ㎛. The substrate was coated on TiO₂ thin films using hydrothermal synthesis at pH 1.5-2.5 at 75℃. The Fe precursor solution was prepared by mixing FeSO₄ (for Fe²⁺ ion) and FeCl₃ (for Fe³⁺ ions) with different molar ratios such as 1/2, 1/1, 2/1, 3/0, and Fe₃O₄ only. X-ray diffraction analysis shows that the crystal structure depends on the FeCl₃-to-FeSO₄ molar ratio. Fe₃O₄ crystal phase could be obtained at higher FeSO₄ contents.