S defect-rich ultrathin 2D MoS2: The role of S point-defects and S stripping-defects in the removal of Cr(VI) via synergistic adsorption and photocatalysis

In the field of photocatalysis , one focus is on high-performance visible light catalysis. For this study, which follows the defect engineering strategy, ultrathin two-dimensional (2D) S defect-rich MoS 2 nanosheets were created in situ by ball-milling MoS 2 nanosheets with ascorbic acid and then used for the removal of Cr(VI) from wastewater. The results show that ascorbic acid increases both the specific surface area of MoS 2 nanosheets and the concentration of S stripping-defects significantly. Of the samples, D-MoS 2 -3 (i.e., S defect-rich ultrathin 2D MoS 2 nanosheets) exhibited the best Cr(VI) adsorption capacity and photocatalytic activity thanks to its large specific surface area and a high concentration of total S defects (18.5%), 311.1% better than for P-MoS 2 (i.e., pristine MoS 2 nanosheets) (4.5%). The concentration of S point-defects in D-MoS 2 -3 is only a little greater than in P-MoS 2 , but the concentration of S stripping-defects is significantly greater. S point-defects at such a high concentration readily act as recombination centers for photogenerated carriers. By contrast, S stripping-defects that lack dangling Mo-S bonds trap photogenerated holes and add to the separation efficiency of photogenerated electron-hole pairs. As a consequence, the photocatalytic performance of D-MoS 2 -3 in removing Cr(VI) is significantly better. Given this finding, the present study offers a new design pathway and a reference for the practical application of defect engineering to ultrathin 2D materials.