Layer-by-layer assembly aids in the creation of artificial photosynthesis

Water oxidation – the conversion of water into oxygen and hydrogen by breaking the oxygen-hydrogen bonds – is one of the most important steps in photosynthesis. It is a difficult process to induce artificially, but it can be streamlined with the help of chemical approaches. As a step toward artificial photosynthesis, Wang et al. demonstrated a layer-by-layer approach to creating photoanodes for water oxidation cells.

“Water oxidation is hard and takes a considerable energy input, in this case from the sun interacting with molecules on an electrode surface,” said author Thomas Meyer.

Using atomic layer deposition (ALD), the group created alternate layers of an electron acceptor chromophore – a light absorber – with an external catalyst for water oxidation on a conductive electrode. The final assembly had a high surface area to ensure sufficient visible light absorption for its use as a photoanode in a water-splitting cell.

In the photoanode, the chromophore absorbs solar photons, placing the chromophore into an excited state. This leads to the transfer of electrons to the electron acceptor and the electrode, and after oxidation, the transfer of holes to the catalyst, which makes oxygen. Water oxidation is the slowest step in the process, with the best device having an incident solar photon-to-current efficiency of 2.2%.

The method serves as a proof-of-concept that ALD design of molecular assemblies can lead to solar water oxidation. With proper redesign and a second photoelectrode for water or CO₂ reduction, an efficiency of 10% may be achievable, opening the door to a solar fuel approach to solar energy conversion.

“Although progress has been made, we have a long way to go,” Meyer said.

Source: “Chemical approaches to artificial photosynthesis. A molecular, dye-sensitized photoanode for O₂ production prepared by layer-by-layer self-assembly,” by Degao Wang, Byron H. Farnum, Christopher J. Dares, and Thomas J. Meyer, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0007383 .