The prediction of a 2-fold increase in demand for fossil fuels over the next 35 years in combination with the damaging consequences of their consumption for the environment has forced scientists and engineers to look for more sustainable means of electricity generation. Amongst all the possible sources of renewable energy it is only solar power that has the potential to meet the predicted increases in energy consumption provided that the issue of solar intermittency can be successfully addressed. Therefore effective ways of storing solar energy are required. A promising approach is the synthesis of chemical fuels such as hydrogen using semiconductors in a photoelectrochemical (PEC) cell.

WO3 is regarded a promising photoanode material performing the water oxidation reaction in a PEC cell. A major limitation of WO3 however, is its ability to only capture a small fraction of the solar spectrum. Amongst a number of different strategies, the decoration of a semiconductor (such as WO3) with metal nanoparticles (MNPs) has been shown to successfully extend the light absorption range of the semiconductor through the excitation of resonant charge oscillations on the MNPs. At the same time, structuring the (host) semiconductor in the form of a photonic crystal (i.e. with a periodically varying refractive index) has been shown to enhance the light absorption in the semiconductor through the slow photon effect. Combining both of these effects (i.e. resonant charge excitations on MNPs and slow photon effect in photonic crystals) in a single structure has been shown to improve synergistically the light absorption capabilities of the semiconductor.