Hidden detail in the natural world could hold the key to future sources of clean energy. So say materials scientists who have created an artificial leaf that can harness light to split water and generate hydrogen.
Plant leaves have evolved over millions of years to catch the energy in the sun’s rays very efficiently. They use the energy to produce food, and the central step in the process involves splitting water molecules and creating hydrogen ions.
By mimicking the machinery plants use to do this, it is possible to create a miniature hydrogen factory, says Tongxiang Fan of The State Key Laboratory of Metal Matrix Composites at Shanghai Jiao Tong University, China. “Using sunlight to split water molecules and form hydrogen fuel is one of the most promising tactics for kicking our carbon habit,” he says.
The idea is not new, but until now researchers have focused on trying to modify or mimic the molecules directly involved in splitting hydrogen. “We’d like to adopt an entirely different concept, to mimic photosynthesis by copying the elaborate architectures of green leaves,” Fan says.
Fan and his colleagues used several types of leaves as a template, including the grape-leaved anemone (Anemone vitifolia). First, they treated the leaves with dilute hydrochloric acid, allowing them to replace magnesium atoms – which form a crucial part of plants’ photosynthetic machinery – with titanium (see illustration).
Then they dried the leaves and heated them to 500 °C to burn away most of the remaining plant material. This left a crystallised titanium dioxide framework plus many of the leaves’ natural structures. Titanium dioxide is commonly used in solar cells to enhance their efficiency, and in the leaf it catalyses the splitting of water molecules.
The leaf retained features such as the lens-like cells at its surface, which catch light coming from any angle, and veins that help guide light deeper into the leaf. The replicas also captured very fine detail, including structures called thykaloids, which increase the surface area available for photosynthesis and are just 10 nanometres thick.
It is these features which make the artificial leaves so efficient at generating hydrogen, Fan says. The team immersed the artificial leaves in a solution containing 20 per cent methanol – which acts as a catalyst – and zapped them with near-ultraviolet visible light. Compared with a commercially available form of titanium dioxide called P25 that can be used to create hydrogen, the artificial leaves absorbed more than twice as much light, and gave off more than three times as much hydrogen, Fan reports (Advanced Materials, DOI: 10.1002/adma.200902039).
The work is a “good beginning”, says Chinnakonda Gopinath of the National Chemical Laboratory in Pune, India. “Complex structures found in leaves should be utilised further for enhancement in light harvesting.”
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