Molybdenum Disulfide Nanodisks For Photoelectrochemical Hydrogen Evolution
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Current strategies of energy production and conversion continue to emit CO2 at a rate that is extremely likely to warm the planet 1.5°C by 2052, and energy sourced from renewables needs to increase 95% by 2050 in the most relaxed reductions emissions scenarios. Photoelectrochemical (PEC) hydrogen evolution from MoS2/p-Si is introduced as a technology that can directly convert solar energy to chemical energy without the use of rare earth metals. The mechanisms of both hydrogen evolution from 2H-MoS2 edge sites and semiconductor photocatalysis are discussed. Colloidal lithography using masks of polystyrene (PS) nanospheres, electron-beam deposition, and chemical vapor deposition were used to control the diameter (200–500 nm), thickness (1.5–9.5 nm) and areal density (1.8–20.9%) of MoS2 nanodisks on p-Si, which were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, and image processing techniques. The PEC performances of bare p-Si and MoS2/p-Si were analyzed using linear scan voltammetry (LSV) and chronoamperometry (CA). Replacing Ga-In with Au as the back contact for p-Si in PEC testing was found to reduce the magnitude of the overpotential at -10 mA cm-2 from -593 to -390 mV due to Schottky barrier removal. 200 nm MoS2 and 500 nm MoS2 nanodisks on p-Si further decreased the overpotential at 10 mA cm-2 from -390 mV to -234 and -172 mV, respectively, and produced short-circuit currents of -0.45 mA cm-2 and -0.80 mA cm-2, respectively. The stability of MoS2/p-Si photocathode performance was found to depend on the thickness of e-beam deposited Mo, with a 31 and 134 mV decrease in overpotential measured for 500 nm MoS2 nanodisks produced from 1 nm and 4 nm Mo, respectively. Finally, the in situ observation of hydrogen evolution from bare p-Si was demonstrated, and images were collected bubbles on the microscale. Future work involves optimizing the thickness of MoS2 to meet state-of-the-art performance parameters and investigating the conditions under which the growth of insulating SiO2 affects photocathode performance.