Artificial Photosystem I and II: Highly Selective Solar Fuels and Tandem Photocatalysis

Artificial photosynthesis, or generation of solar fuels from carbon-dioxide and water, can provide an important alternative for rising carbon-dioxide emission and renewable energy generation. The photosynthesis process performed by plants in an energetically frugal, albeit inefficient process, provides important design principles for success: separation of Photosystem II (PS680) and I (PS700) process, focus on selectivity of product over high efficiency (to save energy in separations), and optimizing the light-harvesting strategies. Here we show novel composite photocatalysts made from widebandgap nanotubes (artificial photosystem II) and different QDs (artificial photosystem I) to separate photo-oxidation and photo-reduction processes and improve material stability. By tuning the redox potentials using the size, composition and energy band alignment of QDs, we demonstrate highly selective (>greater 90%) and efficient production of ethane, ethanol and acetaldehyde as solar fuels, using different wavelengths of light. We also show that this selectivity is a result of precise energy band alignments (using cationic/anionic doping of nanotubes, QD size etc.), confirmed using measurements of electronic density of states. Furthermore, using hot-carriers in these quantum-confined materials, the selective fuels were changed to higher redox potential solar-fuels, using higher energy photons in a novel hot-carrier photocatalysis process. This novel color-selective production of specific solar fuels provides a new pathway to utilize broad-band solar radiation efficiently, improve selective generation of inexpensive solar fuels (like alkanes, alcohols, aldehydes and hydrogen), and all design of tandem structures (red, green, blue) from stacked selective catalysts, allowing almost full visible spectrum (410 ∼ 730nm) utilization with different desired fuels produced simultaneously.