Oxygen Nucleation of MoS2 Nanosheet Thin Film Supercapacitor Electrodes Resulting in Enhanced Electrochemical Charge Storage Performance.

A direct thin film approach to fabricate large surface MoS 2 nanosheet thin film supercapacitors using the solution-based diffusion of thiourea into an anodized MoO 3 thin film is investigated. A dense MoS 2 nanosheet thin film electrode (D-MoS 2 ) was obtained when the anodized MoO 3 thin film was processed in a low thiourea solution concentration whereas a highly porous MoS 2 nanosheet thin film electrode (P-MoS 2 ) was formed at a higher thiourea solution concentration. The charge storage performances of the D-MoS 2 and P-MoS 2 thin films displayed an unusual increase in capacitance on extended cycling, leading to a capacitance as high as ~5-8 mF cm -2 . XRD and cross-sectional microscopy revealed the capacitance enhancements of the MoS 2 supercapacitors are attributable to the nucleation of a new MoS 2-x O x phase upon cycling. For the D-MoS 2 nanosheet thin film, the formation and growth of the MoS 2-x O x phase during cycling was accompanied by a volumetric expansion of the MoS 2 layer. For the P-MoS 2 thin film, the nucleation and growth of the MoS 2-x O x phase occurred in the pores of the MoS 2 layer. The propagation of the MoS 2-x O x phase also shifted the charge storage process in both films from a diffusion-limited process to a capacitive-dominant process.