Engineering the Surface of a Polymeric Photocatalyst for Stable Solar-to-Chemical Fuel Conversion from Seawater

The design of an efficient and highly selective organic polymeric semiconductor photocatalyst consisting of Earth-abundant elements for solar fuel generation using seawater, and also deionized water, as a proton source is reported. The mesoporous g-C₃ N₄ synthesized using a conventional precursor (urea) shows significant H₂ generation (ca. 33 000 μmol h^-1  g^-1 ) and drives the photoreduction of CO₂ to CH₄ , along with trace amount of methanol. However, when the chosen precursor cyanamide is used, drastic improvement in H₂ generation (ca. 41 600 μmol h^-1  g^-1 ) and CO₂ photoreduction is observed. The introduction of a surface nitrogen deficiency and modification of the surface with Cu⁰ further enhances solar H₂ generation (ca. 50 000 μmol h^-1  g^-1 ) and CO₂ photoreduction (3.12 μmol h^-1  g^-1 ) activity, respectively, owing to improvement in light harvesting and charge separation, as revealed by a shorter average lifetime of 3.52 ns and higher Stern-Volmer quenching constant value of approximately 11.2 m^-1 . In addition, improved selectivity in CO₂ photoreduction to only CH₄ is also observed. The designed photocatalytic system is stable, with the solar H₂ generation rate increasing even after 20 h under continuous illumination with a turnover number of 6500. When seawater used instead of deionized water, the overall solar fuel generation efficiencies of all photocatalysts marginally decreased owing to a decrease in the photogenerated charge-carrier separation efficacy.