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 MoS2 nanosheets were created in situ by ball-milling MoS2 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 MoS2 nanosheets and the concentration of S stripping-defects significantly. Of the samples, D-MoS2-3 (i.e., S defect-rich ultrathin 2D MoS2 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-MoS2 (i.e., pristine MoS2 nanosheets) (4.5%). The concentration of S point-defects in D-MoS2-3 is only a little greater than in P-MoS2, 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-MoS2-3 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.