- 10:20 11 July 2008
- NewScientist.com news service
- Colin Barras
Carbon nanotubes are the crucial chemical ingredient that could make artificial photosynthesis possible, say a team of Chinese researchers. The team has found that nanotubes mimic an important step in photosynthesis that chemists have been unable to copy until now.
Artificial photosynthesis has the potential to efficiently produce hydrogen that could be used as a clean fuel for vehicles. It could also be used to mop up carbon dioxide from the atmosphere.
Photosynthetic organisms use the energy from light to break down water into oxygen and hydrogen. The hydrogen then reacts with carbon dioxide to help synthesise carbohydrates, the molecules organisms use to store energy.
Chemists have long tried in vain to reproduce the process, but one key step in particular has proven impossible to copy.
Visible photons can only contribute a limited amount of energy towards a chemical reaction. This energy is absorbed by electrons involved in the reaction.
Reactions that require more energy, such as the synthesis of carbohydrates, can only proceed when several energised electrons are available to contribute. For that reason, chemists say the photosynthesis falls into a class of reactions known as multiple electron systems.
But nobody has succeeded in making artificial multiple electron systems that could provide the necessary energy for artificial photosynthesis.
Such a system would comprise of a donor molecule that can absorb visible light and release many electrons, and a receiver molecule capable of accepting and storing those electrons. Existing systems can donate and receive only one electron at a time.
Now, a team led by Xian-Fu Zhang at the Hebei Normal University of Science and Technology in Qinhuangdao, China, has found that single-walled carbon nanotubes could act as the chemical heart of a multiple electron system.
A carbon nanotube can accept one electron for every 32 carbon atoms it contains, and so even a short nanotube accepts many electrons, says Zhang. That means a carbon nanotube could act as the receiver molecule in artificial photosynthesis.
Although there are no known small molecules capable of releasing a large number of electrons after absorbing visible light, a class of molecule called the phthalocyanines (PCs) does release a single electron when it absorbs light.
Zhang's team realised that by covalently bonding a large number of PC molecules to a carbon nanotube, they could create a multiple electron system activated by visible light.
They found that they could bond 120 PC molecules to a nanotube just 1 micrometer long, and that about 25% of the electrons donated from those PCs end up being stored in the nanotube.
"We decided to create this system initially simply to efficiently convert solar energy into electricity," says Zhang.
But he thinks the nanosystem could form a key component of an artificial photosynthesis model. The extra electrons stored in the nanotubes could be used to convert a chloroplast chemical called NADP into NADPH, which could then reduce carbon dioxide to carbohydrates.
James Barber at Imperial College London, UK, is an expert in photosynthesis. "A lot of people working in this area don't address a basic requirement – that you need to have multiple electrons in photosynthesis," he says. "I think these researchers are right to make this an issue."
Journal Reference: ChemPhysChem (DOI: 10.1002/cphc.200800191)