(326d) Nanostructured MoS2 for Solar Hydrogen Production

The search for promising materials for photoelectrochemical (PEC) water splitting requires the development of photoactive semiconductors with a bandgap large enough to supply the photovoltage necessary for water-splitting, i.e. to drive the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), while being small enough to still absorb a significant portion of the solar spectrum. We approach this challenge by developing nanostructured MoS2 which has previously shown high activity for the HER (1) and quantum confinement of its bandgap (2, 3). Using a reverse micelle encapsulation method (4), we synthesize supported MoS2 nanoparticles in various sizes. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) reveals a controllable range of sizes between 5 nm and 25 nm. Polystyrene(x)-block-poly(2-vinylpyridine)(y) block copolymers are used in the synthesis and are referred to as PS(x)-b-P2VP(y), where x denotes the number of styrene units and y denotes the number of 2-vinylpyridine units per molecule of polymer. The nanoparticles are supported onto inert, electrically conductive electrodes by dip coating. UV-Vis spectroscopy of the supported nanoparticles reveals a size-dependent quantum confinement of the bandgap up to 1.8 eV for the smallest nanoparticles made using the PS(266)-b-P2VP(41) polymer, compared to 1.35 eV for the largest nanoparticles. This is a particularly interesting bandgap range for solar photon harvesting semiconductors. Cyclic voltammetry is used to study the supported MoS2 for the hydrogen evolution reaction (HER). The MoS2 nanoparticles were found to be much more active than single crystal MoS2, likely due to the higher activity of edge sites on the nanoparticles. These nanoparticles produced by a wet chemical method, however, were not quite as active as those previously synthesized in ultra-high vacuum and subsequently studied in an electrochemical cell. We hypothesize that this difference may be due to folding of the MoS2 into fullerene-like structures. Studies of the photoresponse of the nanoparticles revealed that a planar surface arrangement yields too low of a pathlength to adequately absorb enough light to generate photocurrent. To increase optical pathlength, we have synthesized a three-dimensional scaffold consisting of a transparent conducting oxide: indium-tin oxide. MoS2 nanoparticles deposited onto this scaffold have generated a p-type photocurrent under solar-emulated light, indicating light-driven hydrogen evolution on the MoS2. Nanowires of MoS2 have also been investigated for the HER and for photoelectrochemical hydrogen evolution.

References 1. T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 317, 100 (2007). 2. J. P. Wilcoxon, P. P. Newcomer and G. A. Samara, J. Appl. Phys., 81, 7934 (1997). 3. J. P. Wilcoxon and G. A. Samara, Phys. Rev. B, 51, 7299 (1995). 4. J. P. Spatz, S. Mössmer, C. Hartmann and M. Möller, Langmuir, 16, 407 (2000).