The aim of this work was to carry out systematic studies of how synthesis temperatures affect the morphology and properties of mixed zinc sulfide/zinc oxide (ZnSxOy and ZnO) nanostructures, and to get reliable data on optical constants of ZnSxOy and ZnO nanowires/nanobelts (NW/NB) for the use in device applications. ZnSxOy and ZnO NWs/NBs were fabricated using vapor transport in an open-end tube. Mixed ZnS0.47O0.62NWs was obtained at the synthesis temperature of 850 °C. The sulfur content disappeared as the temperature increased to 950 °C and 1050 °C and the morphology changed to a mixture of NW/NB. The NW prepared at 850 °C were indexed as mixed phases of hexagonal ZnS and hexagonal ZnO structures. The NW/NB prepared at 950 °C and 1050 °C were indexed as pure hexagonal ZnO structures. The thickness, surface roughness, and optical constants of the synthesized nanostructured samples were extracted from measurements of spectroscopic ellipsometry. A two-layers model was proposed to fit the calculated data to the measured ellipsometric spectra. The estimated band gap values of the prepared nanostructures lay 0.66–0.79 eV below the bulk ZnO value due to the lower oxygen content present in the samples and the stresses built in the samples during preparation.
