The increasing need for the development of high-performance flexible electronics and optoelectronics has driven the researchers to seek for 2D semiconductor material. Tungsten sulfide ($WS_2$) is a 2D semiconductor layered material with a direct bandgap of 1.9 eV. $WS_2$ has promising optical, electrochemical, and electronic properties with better stability. Besides, modulating the bandgap of $WS_2$, we can tune the electrical and optical properties of $WS_2$. However, synthesis of large size, thin layer $WS_2$ flakes, and their bandgap tuning has relatively less studied, despite its great importance.
Here we demonstrate an approach to synthesis in-situ oxidized $WS_2$ flake with high aspect ratio via oxidative intercalation using hydrate salt (Potassium Sodium Tartrate Tetrahydrate salt). By this method, we produced over 100-micrometer lateral size $WS_2$ flake, which is 50 times larger than the reported value of $WS_2$ flakes produced by liquid-exfoliation method. In addition to that, during exfoliation process, bandgap of $WS_2$ widened, due to the in-situ oxidation. Thermal in-situ oxidative intercalation replaces the sulfur atom by the oxygen atom and widens the bandgap from 1.9 eV to ~2.5 eV. This bandgap change of $WS_2$ flakes can be further modulated by additional oxidation and reduction processes. The results demonstrate the promising possibility of using the tungsten sulfide flakes in the wide range of applications such as a photodetector, solar cell, LED, smart windows and many others in future.