More green solutions under the sun

Grace Chua, Straits Times 25 Jul 09;

WHEN people try to find alternative energy sources to combat global warming, solar power is often bandied about.

But current technologies are not very efficient at converting energy from sunlight. So scientists are looking to plants, which solved the efficiency problem millions of years ago.

In a process called photosynthesis, green plants convert sunlight and carbon dioxide into oxygen, water and sugars.

Chemist Chen Hongyu (above) of Nanyang Technological University is trying to mimic that process.

Meanwhile, he is developing new techniques that could have other applications. In plants, a key step in photosynthesis involves using the sun's energy to create a negatively charged electron and a positive particle.

The excited electron starts a cascade of events that eventually turn carbon dioxide into carbohydrates, while the positive particle eventually causes water to be split to make oxygen.

Previously, researchers were able to replicate this process, but the resulting energised chemicals are free-moving and thus quickly react with one another.

Dr Chen's group uses nanotechnology - engineering on the scale of atoms and molecules - to answer this problem.

The team created tiny nanometer- size, self-assembling polymer capsules that can trap and hold electrons to separate them from the positive particles, the same way soap forms a capsule around a speck of grime to separate it from water.

In each capsule, they placed semiconductor nanoparticles that collect light energy and channel it more efficiently into the water-splitting process.

The project is still in progress, Dr Chen says.

But the polymer-capsule work might have biomedical and industrial applications, for example, different materials in a tiny polymer shell could be used to label molecules, or for drug delivery.

And some day, the water-splitting half of the photosynthesis process could be used to collect hydrogen for fuel cells.