Undoped and Mn-doped ZnS nanocrystals (Ncs) have been synthesized by using the hydrothermal method at 200 degrees C. X-ray diffraction (XRD) patterns revealed that the synthesized Ncs have cubic zinc blende structure. Typical SEM images show that undoped and Mn-doped ZnS Ncs are agglomerated in microspheres. Raman spectra informed that the Mn doping has improved the crystallinity of the ZnS Ncs up to the concentration 3%. The optical properties of Mn-doped ZnS Ncs were studied through UV-vis diffuse reflection spectroscopy, photoluminescence (PL), PL decay and PL excitation (PLE) measurements. The optical band gap was found to decrease from 3.59 to 3.35 eV with increasing the Mn doping concentration. PL spectra demonstrate clearly that Mn2+ ions reduce the density of defect in both ZnS lattice and surface. An efficient excitation transfer from the ZnS host to Mn2+ ions is evidenced from PL decays. ZnS:Mn Ncs were found to be good photocatalyst for sunlight degradation of Rhodamine B. The optimum Mn dopant concentration was 3%, above which photocatalytic activity decreased. The degradation reaction obeyed pseudo-first-order kinetics. (C) 2015 Elsevier B.V. All rights reserved.