Electrodeposited MoS2 and WS2 Thin Films for Pseudocapacitance Studies

Abstract

Cathodic electrodeposition of MoS2 (WS2) atop glassy carbon (ITO) was recently reported utilizing a single-source precursor MoS4 2- (WS4 2-) from an aqueous (acetonitrile) electrolyte. Since MoS2 and WS2 are hydrodesulfurization (HDS) catalysts for sulfur removal from hydrocarbons, they immediately react with and desulfurize the electrolytes from which they are deposited, yielding amorphous deposits that contain excess sulfur. However, the desired stoichiometry MX2 can be recovered by high temperature annealing, which removes excess sulfur and forms moderately crystallized and partially oriented thin films, as shown by x-ray diffraction (XRD) studies. This two-step process for deposition of MoS2 and WS2 thin films of controlled thickness facilitates scientific studies of the nature of pseudocapacitance in these materials. Most reports of MoS2 and WS2 pseudocapacitance study nanocomposites with carbon in order to maximize the specific capacitance, but fundamental electrochemical studies are challenging due to the complex topography and surface heterogeneity. The specific capacitance of MoS2 and WS2 thin films with a thickness ranging from 10 nm to 5μm were studied by cyclic voltammetry and galvanostatic charge discharge in several electrolytes, including 1.0 M aqueous Na2SO4, 1.0 M aqueous LiClO4, and 1.0 M LiClO4 in acetonitrile. For both MoS2 and WS2 thin films, the specific capacitance of stoichiometric films obtained by high temperature annealing is 1-2 orders of magnitude higher than the as-deposited amorphous films. The maximum specific capacitance of 1980 F/g is obtained for 10 nm thick WS2 films in 1.0 M aqueous LiClO4, although this is of course arises from the small mass in the denominator. The capacitance per unit area for WS2 films in 1.0 M aqueous Na2SO4 increases with increasing film thickness, and then reaches a plateau value at a thickness of 30-50 nm. This suggests that the active region thickness for charge storage in WS2 is 30-50 nm, which is close to values previously reported for charge storage in RuO2. Since the cation radius of Li+ (0.90 nm) is smaller than that of Na+ (1.16 nm), higher capacitance per unit area is obtained in LiClO4- than in Na2SO4-containing electrolytes. In addition, the capacitance per unit area in LiClO4-containing electrolytes does not reach a maximum plateau value until a much higher WS2 film thickness. Cathodic electrodeposition of Cu-doped MoS2 atop glassy carbon can also be accomplished from an aqueous electrolyte containing 10.0 mM (NH4)2MoS4, 5.0 mM CuSO4, 0.1 M KCl, and 0.50 M KSCN at pH 6.95. Without the presence of thiocyanate (SCN-) as a complexing agent, the cathodic potentials for electrodeposition of Cu and MoS2 are separated by ~700 mV. However, the addition of SCN- shifts the Cu deposition potential in the cathodic direction towards the deposition potential of MoS2. In addition, a cathodic shoulder is observed during cyclic voltammetry experiments that may indicate an induced co-deposition effect. Electrodeposition at the potential of this cathodic shoulder yields MoS2 thin films that contain 1-4 atom% Cu, as determined by energy dispersive x-ray spectroscopy (EDX) and Rutherford backscattering (RBS). As observed for un-doped MoS2 thin films, high temperature annealing is required to obtain moderately recrystallized and partially oriented films. Four-point probe measurements demonstrate that Cu doping reduces the MoS2 resistivity by 10.3x. The capacitance obtained by cyclic voltammetry and galvanostatic charge discharge for Cu-doped MoS2 thin films is 2.5-3.5x higher than that obtained for MoS2 films of the same thickness.