Phase Tuned Originated Dual Properties of Cobalt Sulfide Nanostructures as Photocatalyst and Adsorbent for Removal of Dye Pollutants

Synthesis of cobalt sulfide nanostructures (NS) with tunable phases was achieved by changing the reaction temperature through facile precipitation method. Prepared NS were fully characterized by powder X-ray diffraction, UV–vis spectroscopy, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, selected area electron diffraction, thermogravimetric analysis, and Brunauer–Emmett–Teller analysis. The NS synthesized at 90 °C (CoS) have photocatalytic property, while at 70 °C (Co4S3) they were found to have adsorption activity. This change of property with the tuning of the phases of cobalt sulfide was observed for the first time to our knowledge. On the one hand, organic dyes such as rhodamine B (RhB), methylene blue (MB), crystal violet, and nile blue were subjected to photocatalysis under visible light irradiation at pH 6.8 and were found to degrade organic dyes rapidly and efficiently with only 2% loss of activity after the seventh cycle of reaction. On the other hand, the adsorbent has shown extremely high adsorption efficiency with maximum adsorption capacity for Congo red (3270 mg/g), RhB (1138 mg/g), and MB (629 mg/g). The adsorption process followed the pseudo-second-order rate kinetics and Langmuir adsorption isotherm model. The reusability of adsorbent was found up to eighth cycle without significant loss of activity.