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Leaves teach technical lessons

Photonics Spectra
Jun 2010
Caren B. Les, News Editor, caren.les@photonics.com

Scientists have introduced a design for an artificial inorganic leaf that can capture solar energy and use it efficiently to change water into hydrogen fuel, an environmentally friendly alternative to nonrenewable fossil fuels such as coal, oil and natural gas. Some predict a “hydrogen economy,” perhaps beginning with a broader acceptance of hydrogen-fueled cars – if a cost-effective way to produce hydrogen is devised. Hydrogen, when burned, emits water vapor. In contrast, fossil fuels, when burned, emit carbon dioxide, a greenhouse gas associated with climate change and other environmental concerns.

With their high light-harvesting efficiency, leaves use sunlight to split water into its components, hydrogen and oxygen, in the photosynthetic process. The green plant will produce biohydrogen (NADP, a kind of compound of hydrogen) and oxygen, not a pure hydrogen.

Dr. Tongxiang Fan and his colleagues at State Key Lab of Metal Matrix Composites at Shanghai Jiaotong University have closely observed the natural structure and function of leaves and applied their knowledge to design artificial leaves with enhanced light-harvesting functions to take advantage of the renewable resource of solar energy.

“Using natural leaves as biotemplates, we developed an artificial inorganic leaf by organizing light-harvesting, photo-induced charge separation and catalysis modules (platinum/nitrogen-titanium dioxide) into leaf-shaped hierarchical structures. The enhanced light-harvesting and photocatalytic water-splitting activities are due to the reproduction of the leaf’s elaborated structures and self-doping of nitrogen during synthesis,” Fan said.


A natural leaf and its hierarchical structures (insets) are pictured. Scientists have reported the design for an artificial inorganic leaf. Courtesy of Tongxiang Fan, Shanghai Jiaotong University.


Using titanium dioxide, a single component catalyst, as a prototype, the researchers demonstrated the successful structural design for improved photocatalytic activity based on the structures of biological systems. Titanium dioxide is a photocatalyst for hydrogen production.

They found that the absorbance intensities within the visible light range of artificial inorganic leaf and titanium dioxide increased by 200 to 234 percent, and that the bandgap-absorption onsets at the edge of the UV and visible ranges showed a redshift of 25 to 100 nm, compared with those in titanium dioxide without the biotemplate. Activities of artificial inorganic leaf/titanium dioxide are eight times higher than titanium dioxide synthesized without templates and 3.3 times higher than P25, a commercial catalyst with high activity, Fan said.

“The 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 modern research,” he added.

Actually, all biomass, such as agricultural and algae wastes, could be used as resources for the fabrication of functional materials with photocatalytic water splitting and photocatalytic degradation activities such as artificial leaves, Fan said.

Form and function

Artificial inorganic leaves have structures similar to the natural leaf from the macro- to the micro- and nanoscales. They have the appearance of leaf-shaped inorganic films, with the thickness of tens of micrometers, he said.

For micro- and nanostructures, the artificial leaves replicate the hierarchical structures of natural leaves, including the lenslike epidermal cells, the veins’ porous architectures, the differentiation of leaf mesophyll into palisade and spongy layers, and the three-dimensional constructions of interconnected nanolayered thylakoid cylindrical stacks (granum) in chloroplast.

For functions, the artificial inorganic leaves could harvest UV and part of visible light. They could split water into hydrogen and oxygen in the presence of sacrificial reagents efficiently under UV and visible light irradiation.

Two main challenges facing research in this area are the limited solar energy harvesting, particularly visible light, and the insufficient energy conversion, particularly in the absence of sacrificial reagents, Fan said.

Applications

“The advantage the technique would have over conventional technologies is that it makes full use of solar energy and biomass,” he noted. “Solar energy is inexhaustible and free. So using sunlight to split water molecules and form hydrogen fuel is one of the most promising tactics for kicking our carbon habit. Biomass also is inexhaustible and cheap. Making full use of solar energy and biomass to generate sustainable energy such as hydrogen and to relieve environmental pollution would be of great significance for the global energy crisis and environment pollution.”

One application of the technique is converting solar energy into chemical fuels. This can be divided into two parts: photocatalytic water splitting for hydrogen production and photocatalytic reduction of carbon dioxide into organic fuels. Fan said this is one of the most promising tactics for kicking our carbon habit and could be significant for the global energy crisis and global climate warming.

The other important application is converting solar energy into electricity. The strategy could be used for the production of cost-effective solar cells, of photovoltaics and of photoelectrochemical cells based on the leaf model.

Renewable hydrogen and organic fuels, such as methanol and ethanol, could be a huge industry in the future. They could be used in cars and mechanical machines – even in homes for daily use, Fan said.

Photonic tools

In the study, several advanced spectroscopic techniques were used for measurement: UV-visible absorption spectroscopy, for the quantitative comparison of overall visible light absorbance intensity and bandgap absorption onsets of samples; electron paramagnetic resonance spectroscopy, for the detection and identification of free radicals and paramagnetic centers, which was used as a preliminary study on the catalytic activity; and x-ray photoelectron spectroscopy, for the confirmation of elemental composition and the chemical state.

Several optics-based tools were used for the characterization: a digital microscope for characterization of surface details such as morphologies and colors of leaves under high magnification; an optical microscope for the observation of the interior structures and colors of leaves; and confocal laser scanning microscopy for the identification of tissues of natural leaves, including cuticles, mesophyll cells, bundle sheath cells and vascular bundles. The latter also was used for the quantitative measurement of fluorescence intensities, which could provide some indications for the chemical procedures. Transmission electron, field-emission and scanning electron microscopes were important for the characterization of the micro-/nanostructures of the natural/artificial leaves.

Further research

“For the next step, we are planning to use a single chloroplast as a biotemplate. The synthesis of artificial chloroplasts with similar structures and analogous functions would be very attractive and interesting,” Fan said.

The production of other series of artificial leaves, including titanates, niobates, tantalates, metal nitrides and phosphides, metal sulfides and other transition metal oxides for higher photocatalytic efficiency, is of interest to the researchers, as is the construction of multicomponent systems for overall water splitting.

The design and construction of a photocatalyst system for the reduction of carbon dioxide into organic fuels by mimicking the dark reaction of photosynthesis would be of great significance for the energy crisis and global climate warming, Fan said.

Finally, the method could be extended to artificial polymeric or supermolecular leaves, which could respond to visible light and which are much closer to natural systems, he added.

Waste not

The investigators also are researching the synthesis of other photocatalysts with good photocatalytic water splitting and photocatalytic degradation activities by using biomass, including agricultural wastes – straws, rice hulls – and ocean algae wastes as the resources.

“This could relieve the increasing energy shortage and environmental pollution. The core idea is ‘governing wastes with wastes.’ This means that the development of novel materials fabrication techniques with agricultural wastes as the resources could be used to relieve the problems of energy shortage and environmental pollution,” Fan said.


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