(530d) Nanostructured MoS2 for the Photoelectrochemical (PEC) Production of Hydrogen
A number of materials criteria must be satisfied simultaneously for photoelectrochemical (PEC) hydrogen production to proceed efficiently and cost-effectively [1,2]. Despite significant research into a number of different materials systems, there has yet to be a semiconductor developed with the appropriate band structure and surface properties for cost-competitive solar PEC fuels [1,2]. Transition metal dichalcogenides (e.g. MoS2) have been investigated previously for PEC hydrogen production [3-5]; it was found that their characteristically small bandgaps (1.0-1.5 eV) were promising for solar photon absorption, but the resulting small photovoltage was found to be too small for water-splitting. Band edge alignment with the redox potentials for water-splitting was also problematic.
In this paper, we present a new approach to engineering semiconductors for PEC. We aim to widen the bandgap of MoS2 through quantum confinement effects. We have successfully synthesized uniform MoS2 nanoparticles of various sizes (3-15 nm), supported onto transparent conducting oxide electrodes. Monodisperse MoO3 nanoparticles were prepared as a precursor to MoS2 using a reverse micelle encapsulation method. We developed a low temperature sulfidization process using H2S/H2 that enables sulfidization at temperatures as low as 150ºC, which mitigates nanoparticle sintering. X-Ray Photoelectron Spectroscopy (XPS) studies show that the sulfidized nanoparticles are completely stable when exposed to air and do not spontaneously re-oxidize, even at the surface. The supported nanoparticles were then characterized opto-electronically using UV-Vis spectroscopy The nanoparticles exhibit a size dependent blueshift in both the direct and indirect band gaps relative to each other and to bulk films, with increases of approximately 1.0 eV to obtain materials with bandgaps in the targeted region of 1.8-2.2 eV. These new PEC materials were then characterized as electrocatalysts for the hydrogen evolution reaction (HER) and photoelectrochemically for photon-driven hydrogen production. The physico-chemical phenomena occurring on and within these nanostructured dichalcogenide semiconductors will be explained; prospects and challenges for this general approach of exploiting quantum confinement effects in nano-scaled semiconductors for PEC will be addressed.
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