Scalable synthesis of two-dimensional molybdenum disulfide (MoS2) via chemical vapor deposition (CVD) is of considerable interests for many applications in electronics and optoelectronics. Here, we investigate the CVD growth of MoS2 single crystals on sapphire substrates by using thermally evaporated molybdenum trioxide (MoO3) thin films as molybdenum (Mo) source instead of conventionally used MoO3 powder for co-evaporation synthesis. The MoO3 thin film source provides uniform Mo vapor pressure in the growth chamber resulting in clean and reproducible MoS2 triangles without any oxide or oxysulfide species. Scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy and atomic force microscopy characterization were performed to characterize the growth results. Very high photoluminescence (PL) response was observed at 1.85 eV which is a good implication of high optical quality of these crystals directly grown on sapphire substrate.