Hydrogen could be an environmentally friendly alternative to conventional fossil fuels, particularly if it is electrochemically produced from ordinary seawater in photoelectrochemical cells (PECMs). However, before this can happen, scientists need to make advanced catalysts that increase the efficiency of the HER. Today, the most efficient HER catalysts are those made from platinum-group metals, but these are expensive.

Dichalcogenides, which have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is S, Se and Te, are easily processed semiconducting films. The materials go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers – a property that could be ideal for making a variety of optoelectronics device applications such as light-emitting diodes and solar cells. They might also be ideal in low-power electronics circuits, low cost or flexible displays, high-performance sensors and even flexible electronics that can be coated onto a wide variety of surfaces. The new results from the Stanford team, confirms that they could be good, inexpensive catalysts for the HER too.

Edge sites are the catalysts

The researchers, led by Yi Cui, have succeeded in synthesising MoSe2 and WSe2 nanofilms on curved and rough surfaces, such as nanowires and microfibres, for the first time. The molecular layers lie perpendicular to the substrate surface – something that exposes the edge sites of the materials, which then act as catalysts. The researchers employed a rapid selenization process to turn Mo and W films into MoSe2 and WSe2 nanofilms, a technique that overcomes the high surface energy barriers of the edge sites in the materials.

"At the high temperatures employed in the selenization process, the Se vapour diffusing along the material layers through van der Waals gaps is much faster than that across the layers," explains Cui. "The layers thus naturally orient perpendicular to the substrates they are grown on and their edges become exposed."

With the molecular layers standing vertically on the substrates, the catalytic activity of the 2D dichalcogenides is boosted since the edges of these layers materials are known to be the active centres for the HER, he told nanotechweb.org. "The curved and rough substrates may not only increase the surface area of the chalcogenides but also squeeze or expand the molecular layers to change the materials’ electronic properties and effectively tune their reaction barriers."

The result is that the overall HER performance of MoSe2 and WSe2 is much improved compared with the corresponding materials on flat substrates.

Solar water splitting applications

Hybrid structures of MoSe2 on silicon nanowires, for example, might be used in applications such as "solar water splitting" (in which water is separated into oxygen and hydrogen using sunlight), adds Cui. Si would act as the light absorber here and would help MoSe2 reduce the overpotential needed to produce hydrogen - that is the extra energy that has to be supplied to the system to kick-start the reaction. Another advantage of WSe2 nanofilms is that they can be used as both photoanode and HER catalyst in PECMs.

"Successfully growing MoSe2 and WSe2 nanofilms on curved and rough surfaces with molecular layers vertical on the substrates opens up the possibility for us to synthesize transition metal chalcogenides as active HER catalysts on 3D structures and enhance the overall activity of these catalysts," said Cui. "We now plan to try and tune the electronic structures of the catalysts using different methods to better understand how the HER actually works and boost HER catalytic activity in the materials we are studying."

The research is detailed in Nano Letters DOI: 10.1021/nl401944f.

Further reading

Nanoparticles for hydrogen production (Apr 2011)
Nanowire coating benefits solar energy conversion (Jul 2012)
Nanoshells improve solar cells (Feb 2012)
Metal nanotrough networks make good transparent electrodes (Jun 2013)