Artificial inorganic leafs for efficient photochemical hydrogen production inspired by natural photosynthesis.

Adv. Mater. 2010, 22, 951–956 2010 WILEY-VCH Verlag Gm Using sunlight to split water molecules and produce hydrogen fuel is one of the most promising tactics for controlling our carbon-based energy ‘‘habit’’. Of the various possible methods, nature provides a blueprint for converting solar energy in the form of chemical fuels. A natural leaf is a synergy of elaborated structures and functional components in order to produce a highly complex machinery for photosynthesis in which light harvesting, photoinduced charge separation, and catalysis modules combine to capture solar energy and split water into oxygen and ‘‘hydrogen’’ (in the form of reducing equivalents) efficiently. Thus, the design of efficient, cost-effective artificial systems by the coupling of leaflike hierarchical structures and analogous functional modules under the guidance of the key steps of natural photosynthesis—capture of sunlight photons, electron–hole separation with long lifetimes, and energy transduction into hydrogen—would be a major advance in the development of materials for energy conversion. Here, we present a general strategy to assemble man-made catalysts (Pt/N-doped TiO2) into leaf-shaped hierarchical structures, named artificial inorganic leaf (AIL), for efficient harvesting of light energy and photochemical hydrogen production. This concept may broaden the horizon for the design of artificial photosynthetic systems based on biological paradigms and provides a working prototype to exploit solar energy for sustainable energy resources. Many research efforts have been carried out to develop artificial photosynthetic systems by constructing a variety of analogous molecular systems consisting of electron donors and acceptors to mimic light-driven charge separation, which occur in photosynthetic reaction centers, or by assembling semiconductor photocatalysts into various nanostructures. Though significant progress has been achieved, most research only focused on the functional imitation of photosynthesis and neglected the structural effect. Actually, the whole structure of natural leaves strongly favors light harvesting: the focusing of light by the lenslike epidermal cells, the multiple scattering and absorbance of light within the veins’ porous architectures, the light propagation in the columnar cells in palisade parenchyma acting as light guides, the enhanced effective light pathlength and light scattering by the less regularly arranged spongy mesophyll cells, and the efficient light-harvesting and fast charge separation in the high surface area three-dimensional constructions of interconnected nanolayered thylakoid cylindrical stacks (granum) in chloroplast. Meanwhile, the photosynthetic pigments in chloroplast successfully perform electron transfer and energy transduction. Thus, in order to mimic a photosynthetic system, it may be necessary for an artificial system to both have similar hierarchical structures for efficient light-harvesting and charge-separation-analogous functional modules, which could i) absorb incident photons, generating excited states, ii) transfer this excitation energy to a donor/ acceptor interface, where photochemical charge separation takes place. Meanwhile, such a system should be able to transfer charge away from the interface in order to limit the rate of wasteful recombination reactions, and iii) it should couple the photochemically generated charges to appropriate catalysts for the production of hydrogen. So our approach for artificial photosynthesis is to construct an artificial leaf by copying the complex architecture of leaves, replace the natural photosynthetic pigments with man-made catalysts, and realize efficient light-harvesting and photochemical hydrogen production. We first demonstrate this new concept with N-doped TiO2, a widely used visible-light-responsive photocatalyst for hydrogen production. Recently, there has been a strong interest in doping TiO2 with anions such as N, S, B, P, C, and halogens. [22–25]