Solar energy conversion is significant to future energy needs if current challenges can be overcome. Photocatalysis of water to H2 and O2 is one solar energy technology that could be used if the efficient conversion from light to chemical fuel (hydrogen and oxygen) could be achieved. However, many photocatalysts can only utilise ultraviolet light, degrade under illumination, mediate only oxidation or reduction, or are too costly.
Some metal silicides have a narrow range of band gap energy, allowing the absorption of visible light and they have been suggested as photocatalysts for hydrogen generation without a sacrificial reductant. A report describes inexpensive commercial TiSi2 as a photocatalyst for water splitting operating under visible light. However in our hands we did not observe catalytic formation of hydrogen. We found that TiSi2 undergoes photocorrosion under visible light to give substoichiometric amounts of hydrogen.
There are many reports of graphitic carbon nitride (g-C3N4) acting as a stable visible-light-driven photocatalyst for H2 generation using a sacrificial reductant. A limitation is the surface area of bulk g-C3N4. To increase the active surface area, we have coated carbon nitride (CN) onto macroporous (3DOM) SiO2 and ZrO2 by thermal polymerisation of cyanamide. A range of loadings have been obtained and the products characterised using SEM, TEM, PXRD, BET, FT-IR, UV-vis, SSNMR and, then for the SiO2 composites, tested for photocatalytic hydrogen production. It has been found that the in common with bulk (g-C3N4) the true composition contains hydrogen due to incomplete polymerisation of cyanamide and overall is closer to C2N3H. Nevertheless the composites show excellent activity for hydrogen production which is over 20 times greater in comparison to the bulk material. The sample with 6.6% CxNy by mass shows the best performance for H2 evolution. The increase in activity can largely be attributed to the increase in surface area.
