Many attempts have been made to create synthetic analogues of the natural photosynthesis system. However, the ability to mimic the spatial organization of this system, where the thylakoid membrane acts as a photosynthetic scaffold that captures and distributes light, and separates the oxidative from reductive species, remains a significant challenge. As reported in the October 16, 2012 online edition of Nature Communications (DOI: 10.1038/ncomms2152), X. Wang and co-researchers at Fuzhou University, China, and M. Antonietti at the Max Planck Institute of Colloids and Interfaces, Potsdam, Germany, have addressed this challenge by employing polymerized hollow nanospheres as analogues of the thylakoid membrane of a chloroplast. These hollow carbon nitride nanospheres (HCNSs) were used as a light-harvesting platform for catalyzing hydrogen evolution under visible light irradiation, achieving a 7.5% overall apparent quantum yield.
The hollow nanospheres were fabricated using 220-nm-silica spheres coated with 20–100 nm mesoporous silica shells as templates. The particle size was selected such that it was similar to the optical bandgap of the carbon nitride semiconductor (less than 430 nm). Light harvesting from inner reflections and photonic effects within the nanostructures is therefore maximized. After loading cyanide into the porous shells and annealing to give graphitic-C3N4-silica nanocomposites, the silica was then removed with NH4HF2 to leave hollow carbon nitride vesicles. Extensive characterization confirmed the generation of graphitic C3N4nanostructures.
The photophysics of the HCNSs was investigated using a light-induced H2-evolution assay, employing 3 wt% Pt as the co-catalyst. While all HCNS samples exhibited enhanced photocatalytic activity over bulk Pt/graphitic-C3N4, the HCNSs with the thickest shells of 85 nm displayed the highest H2-evolution rate of 224 μmol h−1. This is a factor of 25 higher than that of bulk Pt/graphitic- C3N4, and is comparable to benchmark inorganic photocatalysts under UV-light irradiation and outperforms those under visible light irradiation.
The researchers said, “Hybrid nanoarchitectures based on finely tuned polymeric carbon nitride capsules provide a valuable platform for constructing highly organized photosynthetic systems for the efficient and sustained utilization of solar radiation after the controlled deposition of a co-catalyst onto the exterior and/or interior surfaces and the construction of a dyadic layer to promote exciton dissociation.”