Visible light driven photoelectrochemical water splitting on metal free nanoporous carbon promoted by chromophoric functional groups

A nanoporous carbon was prepared from sulfonic acid based water soluble polymer. Its direct one step carbonization followed by air oxidation led to a porous material whose surface was rich in N-, O- and S-containing groups. TEM images showed the presence of 10 nm graphitic domains in the carbon matrix. When the carbon was used as a photoanode under visible light irradiation, oxygen evolution occurred at overpotentials as low as +0.8 V vs. Ag/AgCl. The ability of the carbon to oxidize water was linked to the presence of sulfur and nitrogen containing groups working as chromophores whose electrons are excited by irradiation leaving reactive vacancies (holes) that are able to accept electrons from oxygen in water molecules. The conductivity of the carbon matrix and the presence of small pores were found of paramount importance. While the former helps in electron transfer within the carbon matrix, strong adsorption of water molecules in micropores close to chromophores enhances the electron transfer from oxygen to the vacancy on the chromophore promoting water splitting and oxygen evolution. This work provides the first direct experimental evidence of water photooxidation reactions using metal-free nanoporous carbons exposed to visible light.