The sesame ball-like CoS/MoS2 nanospheres as efficient counter electrode catalysts for dye-sensitized solar cells

Highlights: • CoS nanoparticles grew on the MoS₂ nanosheets to get the sesame structure. • CoS/MoS₂ nanospheres exhibited a high power conversion efficiency of 7.48%. • Co atoms were the main catalytic active site for I₃⁻ reduction. In this work, the sesame ball-like CoS/MoS₂ nanospheres were controllably synthesized via a facile hydrothermal and solvothermal consecutive reaction. MoS₂ nanospheres were preferentially formed by assembly of nanosheets during the hydrothermal reaction. And then CoS nanoparticles were grown on the surface of MoS₂ nanosheets during the solvothermal reaction method. Due to the space confinement effect, CoS grew as nanoparticles with size of 5 nm between MoS₂ nanosheets, and the nanosheets assembled into nanospheres to get the sesame ball-like structure. Because this composite possessed two merits, that is, more active sites of CoS nanoparticles and high conductivity of MoS₂ nanospheres, DSSCs based on this sesame ball-like CoS/MoS₂ nanospheres got a higher power conversion efficiency of 7.48%, which was higher than that based on the solo CoS (7.10%) and solo MoS₂ (4.84%). And it was similarly to the DSSCs based on Pt cathode (7.51%). Their excellent electrocatalytic activities for the I₃⁻/I⁻ redox reaction was confirmed by cyclic voltammetry, electrochemical impedance spectra and Tafel polarization measurements. Density functional theory was also used to calculate the adsorption of I₃⁻ on the surface of CoS/MoS₂, and the adsorption energy and bond length results indicated Co atoms were the main catalytic active site for I₃⁻ reduction. The hybrid interactions took place between p orbital of I and 3d states of Co, which lead to I₃⁻ was activated and dissociated easily.