Photocatalytic hydrogen production over layered materials
Keywords: Layered materials
Charge separation.
Photocatalytic activity
Hydrogen production
Semiconductors
Photocatalysts
MoS2
Graphene
CdS

The technology of semiconductor-based photocatalytic water splitting to produce hydrogen using solar energy has been considered as one of the most important approaches to solve the world energy crisis. Therefore, the development of the effective semiconductor photocatalysts has undergone considerable research. However, the traditional photocatalysts suffer from the negative effects from rapid charge recombination, which reduces the excited charges by emitting light or generating phonons. Efficient charge separation and fast charge transport, avoiding any bulk/surface recombination, are fundamentally important for photocatalytic hydrogen generation through water splitting. Here, we have introduced assembled layered materials as photocatalyst systems with their unique physicochemical properties to realize the effective charge separation and high photocatalytic activity.
Using graphene as a two-dimensional supporting matrix, we have succeeded in selective anchoring of semiconductor and metal nanoparticles as separate catalytically active sites on the graphene surface. The ability of graphene to capture, transfer and store electrons and its potential to serve as a conductive support are demonstrated. The TiO2 semiconductor/metals nanocrystals-graphene ensemble makes it possible to carry out selective catalytic processes at the separate sites and provides the potentials for applications in water splitting reactions. After demonstrating the positive role of graphene in such photocatalytic system, we then fabricate a simple but highly cooperative ensemble with CdS and MoS2 nanocrystals dispersed on graphene sheets. It is demonstrated that CdS nanocrystals can also capture visible light energy and facilitate excited electron transfer to MoS2 (as metal substituent) for catalytic hydrogen production via the 2-D graphene which plays a key role as an efficient electron mediator. Hexagonal multilayer MoS2 with a layered structure in this system serves to provide active sites for hydrogen evolution by its exposed Mo edges. Hence, multilayer MoS2 is an ideal cocatalyst of semiconductors for hydrogen generation. This crystalline-layered structure also shows semiconducting properties, however, its characteristic indirect band gap displays a poor light capture and emission ability with excited electrons and holes with different momentum. In contrast, single layer MoS2 shows a direct band gap behavior. Our studies have clearly shown that single layer MoS2 prepared with lithium intercalation indeed displays encouraging results in hydrogen evolution due to the direct band gap and quantum confinement effects. In addition, the exfoliated single layer MoS2 exhibits extraordinary enhanced activity and stability in combination with the Eosin Y sensitized system when compared to those of multilayer MoS2 and bulk MoS2 counterparts, which is attributed to the improvement of the density of surface active sites with stronger adsorption for the Eosin Y molecules on the single layer MoS2. In addition, this multifunctional catalyst on graphene sheet can also create adsorption sites on a defective basal surface of single layer MoS2 through adsorption of Eosin Y where electron transfer from photoexcited Eosin Y molecule to graphene via the 2-D MoS2 mainly takes place. Thus, the photo-generated electrons are then effectively transported to the exposed active sites of MoS2 for the proton reduction to hydrogen molecule. It is believed the above novel assembled molecular layered systems may be applicable for a wide range of catalytic,photocatalytic and electrocatalytic reactions.