Nanotechnology artificial leaves for hydrogen production
(Nanowerk Spotlight) Artificial photosynthesis, using solar energy to split water generating hydrogen and oxygen, can offer a clean and portable source of energy supply as durable as the sunlight. Natural photosynthesis uses chlorophyll to absorb visible light and many solar hydrogen cells are imitating this process by using light-sensitive organic dye molecules as light absorbers and then transfer the absorbed energy to a catalyst that reduces protons to hydrogen (read: "Another step towards inexpensive hydrogen production from sunlight").
Today, over 130 materials and derivatives are known to facilitate photocatalytic splitting of water to produce hydrogen. Many efforts have been made to design new photocatalysts of different materials such as transition-metal oxides or metal oxynitrides or in nanotechnology research to design photocatalysts with various nanoscale morphologies such as nanoparticles, nanosheets, nanowires, etc for enhanced light-harvesting and catalytic efficiency.
"Using sunlight to split water molecules and form hydrogen fuel is one of the most promising tactics for kicking our carbon habit," Di Zhang tells Nanowerk. "Of the possible methods, nature provides the blueprint for converting solar energy in the form of chemical fuels. A natural leaf is a synergy of the elaborated structures and functional components to produce a highly complex machinery for photosynthesis in which light harvesting, photoinduced charge separation, and catalysis modules combined to capture solar energy and split water into oxygen and hydrogen efficiently."
Zhang, a professor at Shanghai Jiao-Tong University in China and director of the university's State Key Laboratory of Metal Matrix Composites, points out that the design of efficient, cost-effective artificial systems by coupling of leaf-like hierarchical structures and analogous functional modules under the guidance of the key steps of natural photosynthesis into hydrogen would be a major advance in energy conversion.
Many efforts have been made to develop such systems by constructing a variety of analogous molecular systems consisting of electron donors and acceptors to mimic light driven charge separation or by assembling semiconductor photocatalysts into various nanostructures. However, most of them only focused on the functional imitation of photosynthesis, and neglected the structural effect.
A leaf and its hierarchical structures. (Image: Dr. Di Zhang, Shanghai Jiao-Tong University)
"Actually" says Zhang, "the whole structures of natural leaves are greatly favorable for light harvesting: the focusing of light by the lens-like epidermal cells; the multiple scattering and absorbing of light within the veins porous architectures; the propagating of light in the columnar cells in palisade parenchyma acting like light guides; the enhanced effective light path length and light scattering by the less regularly arranged spongy mesophyll cells; the efficient light-harvesting and fast charge separation in the high surface are of a three-dimensional constructions of interconnected nano-layered thylakoid cylindrical stacks (granum) in chloroplast."
Consequently, Zhang's team and a group of collaborators from the University of California, Davis and Saga University in Japan, adopted an entirely different concept to mimic photosynthesis by copying the elaborate architectures of green leaves, replacing the natural photosynthetic pigments with man-made catalysts and further realizing water splitting.
Based on the prototype of the hierarchical structures of a natural leaf, the researchers obtained nitrogen-doped titanium dioxide replicas via a two-step infiltration process with natural leaves as templates.
"The replicas inherit the hierarchical structures of the natural leaf at macro-, micro-, and nanoscales including convexly shaped epidermal leaf cells, tubelike parallel bundle sheath extensions, a porous framework of veins, the differentiation of columnar palisade mesophyll cells, and irregularly arranged spongy cells, and nanolayered lamellar structures of granums in chloroplast" explains Zhang. "By reproducing a natural leaf's elaborated structures and self-doping of nitrogen during synthesis we were able to realize enhanced light-harvesting and photocatalytic hydrogen evolution activities."
Scanning electron microscope image of a cross-section of AIL-TiO2 derived from A.vitifolia Buch leaf. (Image: Dr. Di Zhang, Shanghai Jiao-Tong University)
The absorbance intensities within the visible-light range of the replicas increase by 200?234% and the bandgap-absorption onsets at the edge of the UV and visible-light range show a red-shift of 25?100nm compared to those in titanium dioxide without the template. The photocatalytic water splitting activity of the artificial leave structures is 8 times higher than titanium dioxide synthesized without templates.
This research may represent an important first step towards the design of novel artificial solar energy transduction systems based on natural paradigms, particularly on mimicking the structural design. The work could be a real breakthrough suggesting an important (and uncommon) preparation strategy to obtain an active photocatalyst for water-splitting and opening new perspectives in this strategic area of 'green' energy research.
"Our artificial leave is a man-made material that has similar functions than the natural original – light-harvesting, charge separation and redox reactions" says Zhang. "Actually, all biomass, such as agricultural wastes, algae wastes, etc could also be used as a resource for the fabrication of functional materials with photocatalytic water splitting and photocatalytic degradation activities like our artificial leaves."
Going forward, the research team plans to explore using single chloroplasts – the organelles found in plant cells – as biotemplates.
"Chloroplasts are the photosynthetic sites combining photosynthetic pigments and elaborate three-dimensional constructions of interconnected nanolayered thylakoid cylindrical stacks (granum) for efficient light-harvesting and photosynthesis" explains Zhang. "Therefore the synthesis of artificial chloroplasts with similar structures and analogous functions would be very attractive and interesting."
Another area that the researchers are planning to investigate is the fabrication of artificial leaves using titanates, niobates, tantalates, metal nitrides and phosphides, metal sulfides and other transition metal oxides in order to increase the photocatalytic efficiency. Zhang also believes that the construction of multicomponents systems such as TiO2-CdS, MoS2/CdSe, etc for overall water splitting would be very interesting as well.
Finally, the method could be extended to artificial polymeric or supermolecular leaves which could respond to visible light and are much closer to natural systems.
"The study of such artificial systems not only contributes to our understanding of natural photosynthesis, but also aids in the design of novel artificial solar energy harvesting and conversion systems and would provide a blue-print to harness nature's optimization of this process for the development of complex man-made energy devices" concludes Zhang.