Semiconductors can help "green the economy" - An analysis

New pathways to finding green energy have taken man to "double perovskites"

 Semiconductors can help "green the economy" - An analysis

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• The story: In the laboratories of the IIT 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.
• Nature of research: The investigation is about 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 turned into electricity or used to split water into hydrogen and oxygen. The work thus is ‘tuning’ materials at the atomic level to collect electrons.
• The science: In nature most materials are symmetric. Symmetry is beautiful, but asymmetry is useful.

1. 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.
2. If we 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.
3. 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.
4. When one shines 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.
5. But when we 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.

• Specific material: The scientists have 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.
• Unexplored field: The effect of polarisation induced by material distortion is a completely un(der)-explored area in the domain of catalysis. 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.
• Applications: 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. Scientists have demonstrated their use in splitting water to produce hydrogen. When electrons are supplied 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. The distorted materials also find other applications — as bifunctional catalysts in metal-air batteries, carbon dioxide conversion to fuels like formic acid and methane, wastewater treatment, production of ammonia from nitrogen and decontamination of pharma wastes.
• EXAM QUESTIONS: (1) Explain the idea behind 'polarisation photoelectrochemistry'. (2) How are "double perovskites" being used to explore totally new green energy pathways? Explain.

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