Semiconductors aid artificial photosynthesis

Researchers at Caltech’s Joint Center for Artificial Photosynthesis (JCAP) are mimicing photosynthesis using semiconductors to make hydrogen and oxygen from water and carbon dioxide.

Their artificial leaf consists of a photoanode, a photocathode and a membrane.

Light oxidises water molecules to generate oxygen gas, protons, and electrons on the photoanode, while the photocathode recombines the protons and electrons to form hydrogen gas. The membrane keeps the gases separate.

This has been tried before using semiconductors including silicon and GaAs, said Caltech, but water and oxygen at the photoanode oxidise the semiconductor and ruin it.

A team lead by Professor Nate Lewis has experimented with protective anode coatings. “You want the coating to be many things: chemically compatible with the semiconductor it’s trying to protect, impermeable to water, electrically conductive, highly transparent to incoming light, and highly catalytic for the reaction to make oxygen and fuels,” he said.

Now JCAP has found something that can do all of these things at once – a nickel oxide film that is compatible with semiconductors including silicon, indium phosphide, and cadmium telluride.

“After watching the photoanodes run at record performance without any noticeable degradation for 24 hours, and then 100 hours, and then 500 hours, I knew we had done what scientists had failed to do before,” said lab scientist Dr Ke Sun.

Invented by the team, the coating technique involves using argon atoms to break fragments off a nickel pellet in an oxygen-rich environment. The sputtered fragments oxidise and are deposited on the semiconductor as a mixed oxide (NiO_x).

This oxide is also compatible with the cell membrane.

“Without a membrane, the photoanode and photocathode are close enough to each other to conduct electricity, and if you also have bubbles of reactive hydrogen and oxygen gases being produced in the same place at the same time, that is a recipe for disaster,” said Lewis. “With our film, you can build a device that will not explode, and that lasts, and is efficient, all at once.”

Lewis cautions that the team is nowhere near a fuel-making commercial product.

“Our team is working on a photocathode,” he said. “What we have to do is combine anode and cathode and show that the entire system works, and that will not be easy.”

The work is published in the Proceedings of the National Academy of Sciences as ‘Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films‘ which covers testing on: textured crystalline silicon passivated by amorphous silicon, crystalline n-type cadmium telluride, and hydrogenated amorphous silicon.

Last year, Lewis’ group developed a two-layer coating which worked, but was less successful.

Tried on single crystals of silicon, GaAs and GaP, this one consisted of atomic layer deposited ‘leaky titanium oxide’ (TiO₂ that conducts electricity), with 100nm thick islands of nickel oxide on top.

steve bush

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