M Ramesh

In the laboratories of the Indian Institute of Technology, Madras, a new branch of science is evolving. Called ‘polarisation photoelectrochemistry’, it refers to a lab-cooked combination of three areas of science — semiconductors, electrochemistry and photo-physics.

Prof Aravind Chandiran of the Department of Chemical Engineering is leading an investigation into how materials — semiconductors — could be ‘distorted’, or polarised, so that electrons bunch up at one of a molecule, making it easy to eject a few. The footloose electrons could then be gainfully employed. You can stream them as electricity or use them to split water into hydrogen and oxygen.

The heart of the work, currently underway at the Centre for Photoelectrochemistry, one of the centres of excellence under IIT-M, is ‘tuning’ materials at the atomic level to collect electrons.

Chandiran (who quips that he and his team are “distorted people”) explains that in Nature most materials are symmetric. Symmetry is beautiful, but asymmetry is useful.

Imagine a molecule of elements X and A, with X in the middle and two As equidistant on either side — perfect symmetry. The distance between X and one A is the ‘bond length’, measured in ‘angstrom’. (An angstrom is ten-billionth of a metre). Visualise the compound like this: A—X—A.

If you knock off one ‘A’, say on the right, and introduce another element ‘B’, the compound could look like A—X-B. Note that the distance between X and B is shorter than that between X and A. That is asymmetry.

Distortion leads to asymmetric alignment of atoms in a material, and the creation of a dipole (polarisation). A dipole is a molecule where the electrons of its atoms have bunched up at a given location.

Now, when you shine a light on a semiconducting material (such as crystalline silicon), the energy of the light gets the electrons into an ‘excited state’, which is higher than their normal state, like a ball in the air. When this happens, it is easy to pull out the electrons and make use of them — this is how solar PV works.

But when you shine a light on distorted materials, the electrons remain longer in the excited state, giving us more time to rip off the electrons. The duration of the excited state increases in distorted materials to micro- or milli-seconds, compared with nano-seconds in non-distorted materials.

Chandiran has chosen material of a particular structure, called double perovskites (also called ‘vacancy ordered perovskites’) for distortion. “The number of atoms that constitute the double perovskites are fairly high, which enables us to tune more combinations of atoms,” Chandiran told Quantum .

“The effect of polarisation induced by material distortion is a completely un(der)-explored area in the domain of catalysis,” says a background note on Chandiran’s project. “Once the concept is verified in perovskite materials, it can be extended to any class of materials where polarisation shall be induced for any catalytic reaction,” it says.

What is the use?

Such materials tuned at the atomic level can be put to a number of uses, essentially because the electrons can be more easily detached from the atoms. Chandiran’s team has demonstrated their use in splitting water to produce hydrogen. When you supply electrons to water, it splits into hydrogen and hydroxyl molecules. The direct sunlight-to-hydrogen pathway, or photoelectrolysis, as opposed to sunlight-electricity-hydrogen pathway, is not new, but the use of distorted materials (double perovskite) is the uniqueness of Chandiran’s work.

The distorted materials also find other applications — as bifunctional catalysts in metal-air batteries (see ‘Leapfrogging lithium batteries’ above) , carbon dioxide conversion to fuels like formic acid and methane, wastewater treatment, production of ammonia from nitrogen and decontamination of pharma wastes.