CdS crystallites with rod- and flower-like architectures were synthesized using a facile hydrothermal growth method. The hexagonal crystal structure of CdS dominated the growth mechanisms of the rod- and flower-like crystallites under specific growth conditions, as indicated by structural analyses. The flower-like CdS crystallites had a higher crystal defect density and lower optical band gap value compared with the rod-like CdS crystallites. The substantial differences in microstructures and optical properties between the rod- and flower-like CdS crystallites revealed that the flower-like CdS crystallites exhibited superior photoactivity, and this performance could be further enhanced through appropriate thermal annealing in ambient air. A postannealing procedure conducted in ambient air oxidized the surfaces of the flower-like CdS crystallites and formed a CdO phase. The formation of heterointerfaces between the CdS and CdO phases mainly contributed to the improved photoactivity of the synthesized flower-like CdS crystallites.

In this study, ZnO–Sn2S3 core–shell nanorod heterostructures were synthesized by sputtering Sn2S3 shell layers onto ZnO rods. The Sn2S3 shell layers consisted of sheet-like crystallites. A structural analysis revealed that the ZnO–Sn2S3 core–shell nanorod heterostructures were highly crystalline. In comparison with ZnO nanorods, the ZnO–Sn2S3 nanorods exhibited a broadened optical absorption edge that extended to the visible light region. The ZnO–Sn2S3 nanorods exhibited substantial visible photodegradation efficiency of methylene blue organic dyes and high photoelectrochemical performance under light illumination. The unique three-dimensional sheet-like Sn2S3 crystallites resulted in the high light-harvesting efficiency of the nanorod heterostructures. Moreover, the efficient spatial separation of photoexcited carriers, attributable to the band alignment between ZnO and Sn2S3, accounted for the superior photocatalytic and photoelectrochemical properties of the ZnO–Sn2S3 core–shell nanorod heterostructures.

Well-crystallized Sn2S3 semiconductor thin films with a highly (111)-crystallographic orientation were grown using RF sputtering. The surface morphology of the Sn2S3 thin films exhibited a sheet-like feature. The Sn2S3 crystallites with a sheet-like surface had a sharp periphery with a thickness in a nanoscale size, and the crystallite size ranged from approximately 150 to 300 nm. Postannealing the as-synthesized Sn2S3 thin films further in ambient air at 400 °C engendered roughened and oxidized surfaces on the Sn2S3 thin films. Transmission electron microscopy analysis revealed that the surfaces of the Sn2S3 thin films transformed into a SnO2 phase, and well-layered Sn2S3–SnO2 heterostructure thin films were thus formed. The Sn2S3–SnO2 heterostructure thin film exhibited a visible photoassisted room-temperature gas-sensing behavior toward low concentrations of NO2 gases (0.2–2.5 ppm). By contrast, the pure Sn2S3 thin film exhibited an unapparent room-temperature NO2 gas-sensing behavior under illumination. The suitable band alignment at the interface of the Sn2S3–SnO2 heterostructure thin film and rough surface features might explain the visible photoassisted room-temperature NO2 gas-sensing responses of the heterostructure thin film on exposure to NO2 gas at low concentrations in this work.

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