The search for suitable semiconductors as photocatalysts for the splitting of water into hydrogen gas using solar energy is one of the noblest missions that face material scientists today. With the continual depletion of the world's energy resources, scientists are charged with finding alternative sources to continue the fuelling of our economy. The cold fusion debacle inititated the general publics interest in the ideal of generating energy from common elements around us. Since then research has continued with some notable results. The production of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of our precious fossil reserves.
From a feasibility and economical perspective when relying solely on water and solar energy, catalysts that are sufficiently stable, inexpensive and capable of generating light are required. A group of research scientists in Germany, China and Japan, [Wang et al., nmat. 2008 DOI: 10.1038/NMAT2317] has shown that a commercially viable, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of what is known as a sacrificial donor. Unlike other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated manufacturing methods. The results mark an important first step towards photosynthetic pathways where artificial conjugated polymer semiconductors can be used as energy transducers.
An optimal material would combine an ability to dissociate the water molecules, having a bandgap that absorbs light in the visible range and to remain stable in contact with water
The team showed that another simple polymer-like semiconductor, made of only carbon and nitrogen, can function as a metal-free photo-catalyst for the extraction of hydrogen from water.
The photocatalysis experiments were carried out with C3N4 powder to ensure sufficient surface area. The C3N4 achieved steady H2 production from water containing triethanolamine as a sacrificial electron donor on light illumination. Even in the absence of noble metal catalysts such as Pt. These results indicate that C3N4 functions as a stable metal free photocatalyst for visible light driven H2 production.
The H2 production rate increases with increasing Pt content to a plateau at around 2–4%, beyond which it decreases again.
In summary the researchers have shown polymeric carbon nitride as a commonly available and simple photocatalyst that is able to generate hydrogen from water even in the absence of noble metals.
Work is continuing within each of the research institutions to improve upon the H2 production mechanism.