The direct photolysis of water for hydrogen production has been deemed a Holy Grail of Chemistry.¹  First shown in 1972 with TiO₂, the photoelectrochemical splitting of water has been a dream through the decades. While the process seems to be the simplest and most straightforward approach for solar-driven water splitting, ultimately though there has not been a large-scale demonstration of a photoelectrochemical water splitting system.

For a commercially viable system, the key parameters are solar-to-hydrogen (STH) efficiency, system cost and lifetime, of which the efficiency carries the greatest weight.²  This is not unexpected, since land area must be covered and that relates directly to the capital cost of the system. Unfortunately, the main focus of research over the decades has been on oxides due to their expected low cost and stability. In general oxide semiconductors have very poor solid-state characteristics,³  thus their STH efficiencies remain very low.