Timing of sulfur introduction in the sulfurization of WO3 films dictates WS2 formation

[Display omitted] •Timing of S introduction play a decisive role in the synthesis of WS2 films.•A threshold S concentration is necessary to an efficient sulfurization of WO3.•Structural and morphological modifications of WO3 change its reactivity toward S. Sulfurization of WO3 films can be an alternative to conventional chemical vapor deposition processes were both WO3 and S must be transported in the gas phase. Nevertheless, W loss and incomplete sulfurization were reported using this approach, constituting experimental hurdles to obtain WS2 layers with a specific thickness. The moment of S introduction in the process showed to play a decisive role, determining the obtention of a completely sulfurized layer or a modified oxide. Results evidenced that during a certain period, while sulfurization conditions are not efficient (S heating zone below 170 °C), WO3 annealing induces morphological and structural modifications (mainly crystallization) which modify oxide’s reactivity towards S. With this knowledge, efficient sulfurization strategies can be designed aiming at obtaining WS2 layers with the desired characteristics.