The reliable synthesis method of large-area, continuous 2D MoS2 layer has been demanded for applications of next generation electronic and optoelectronic devices. Here, Na2S is introduced as a novel growth promoter to control over the grain size of full area coverage MoS2 monolayer. With Na2S solution-coated film, the nucleation density increases by the formation of reduced phase like NaMoO3-x. At the same time, additional sulfur from Na2S film affects the nucleation and growth mechanism resulting in changes of the domain geometry from triangular nanosheets to centimeter scale films. Moreover, by modulating the concentration of Na2S solution, the grain size of MoS2 monolayer has been changed. Thus, we propose a new approach of modulating nucleation density and grain morphology in polycrystalline full-coverage MoS2 films by using growth promoter, Na2S. The successful large-scale monolayer has been characterized for electromechanical energy conversion. As a result, a peak output of 253mV and 22nA was obtained when a strain of 0.26% was applied. Our results suggest the potential of large-scale 2D layer as an electromechanical energy converter.