Metal chalcogenide related materials have demonstrated outstanding catalytic activities for numerous chemical reactions in energy conversion devices. However, the actual active species have not yet been fully identified, especially for triiodide reduction in dye-sensitized solar cells (DSSCs). We herein adopted a vapor-phase hydrothermal (VPH) approach to controllably create the sulfur dopants on the Co3O4 nanosheets surface for high-performance DSSCs with a best power conversion efficiency (PCE) of 8.86%. More importantly, our XPS and electrochemical analysis confirm a close correlation between the S2− species and the electrocatalytic activities.