Cluster Size Effects in the Photocatalytic Hydrogen Evolution Reaction

In  Germany, the photocatalytic water reduction reaction on CdS nanorods was studied. Maximum number of hydrogen was produced for Pt46. IT is due to the size dependent electronic properties of the clusters with respect to the band edges of the semiconductor. Beacuse control of particle shape and size results in control over their electronic optical properties. The clusters were evenly distributed over the nanorods and the size of the clusters were significantly different from each other. In the experiment, the various clusters ranged from about 0.5nm^2 for Pt8 to over 1.0nm^2 for Pt68. 

The average H2 production rate changed with the size of the clusters deposited. These quantum size effects are particularly strong for clusters with a few tens of atoms.

If a photon energy larger than the band gap is illuminated, an electron pair is created in the semiconductor. In order for the H2 gas to be formed with the electron created from the photon, the electron has to be trapped efficiently at the cluster and then transferred to the protons. For the efficient trapping, its Lowest Unoccupied Molecular Orbital has to be lower in energy than the lower edge of the conduction band of the semiconductor.