Electrodeposition and Capacitance Measurements of WS2 Thin Films

Abstract

Polycrystalline WS2 thin films were fabricated by electrodeposition onto ITO from an electrolyte containing 4 mM (NH4)2WS4 + 0.1 M LiClO4 in acetonitrile, followed by a two-stage sulfurization and crystallization procedure at elevated temperature in a tube furnace. The as-deposited films had a S:W stoichiometric ratio of ∼2.8, which decreased to 2 after annealing. X-ray diffraction indicates that the annealed WS2 films crystallize into the 2H polytype with an average grain diameter of ∼22 nm. The capacitance of WS2 thin films was studied in different electrolytes, and the maximum capacitance observed was 1980 F/g, which decreased to 1610 F/g after 1000 cycles, for 10 nm thick WS2 in 1.0 M aqueous LiClO4. Capacitance studies as a function of WS2 film thickness yield different behavior in electrolytes containing Li+ and Na+, with the capacitance per unit area plateauing at a considerably higher film thickness in Li+-containing electrolytes. This suggests that the active depth for charge storage increases due to the smaller ionic radius of Li+.