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Light-activated catalysts make nearly perfect water-splitters

Monday, July 6, 2020   (0 Comments)
Posted by: Matt Watson
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Light-activated catalysts make nearly perfect water-splitters


03 Jul 2020 Isabelle Dumé

Using sunlight to decompose water could be a clean and renewable way to produce hydrogen fuel, but the photocatalysts traditionally used to promote the process are relatively inefficient. Researchers in Japan have now developed a model system based on strontium titanate that has an external quantum efficiency of 96%, proving that almost perfectly efficient catalysts are possible.

Because the combustion of hydrogen yields only pure water as a waste product, it is often touted as an environmentally-friendly alternative to fossil fuels. The caveat is that to be truly “green”, the hydrogen itself needs to be produced using renewable energy. Solar water splitting, in which sunlight is directed onto an aqueous suspension of light-activated semiconducting particles, is one way of cleanly producing hydrogen. When these particles absorb solar photons, the resulting electron-hole pairs catalyze the breakdown of water, liberating the hydrogen.

Several roles

The drawback of this method is that the catalytic process is highly complex, requiring the semiconductor particles to play several roles at once. First, they must be able to absorb light in the solar spectrum range, which means they need to have narrow bandgaps near the 500nm peak of the Sun’s emissions. Second, they need to generate and then separate electron-hole pairs. Third, they must allow these holes and electrons to travel to the particle-water interface and catalyse the production of hydrogen (a process that requires electrons) and oxygen (a process that requires holes) from water. Last, but not least, they need to minimize unwanted side processes (which can lower the overall efficiency of the system) occurring at each step along the way.

That is a long list, and although researchers have long been searching for efficient photocatalytic materials, typical photocatalysts have an external quantum efficiency (EQE) – that is, the fraction of photons impinging on the system that end up being used to produce hydrogen – of less than 10%.

Strategies for reducing loss mechanisms

In their work, a team of researchers led by Kazunari Domen of Shinshu University in Nagano and the University of Tokyo focused on strontium titanate (SrTiO3), a photocatalytic water-splitter that was discovered in the 1970s. Although SrTiOis impractical for making real-world photocatalysts (it produces electron-hole pairs by absorbing near-ultraviolet light rather than visible light), the researchers argue that it is nevertheless a good model system because the mechanisms responsible for its efficiency losses are well understood.

Domen and colleagues studied several ways of reducing loss mechanisms in SrTiO3. The first involved suppressing charge carrier recombination, which occurs when electrons and holes recombine before they can take part in the water-splitting reaction. Since defects in the crystal lattice act as potential recombination hubs, the researchers used a flux treatment to improve the crystallinity of the photocatalyst particles, thereby reducing the number of lattice defects. They then reduced the number of chemical defects in the lattice by aluminium doping.

The team’s second strategy was to further suppress charge recombination by taking advantage of the fact that electron and holes in SrTiO3 crystals collect at different crystal facets. They did this by selectively depositing specific co-catalysts on the different facets to enhance hydrogen production at the electron-collecting facets and oxygen production at the hole-collecting ones. Although this approach is not new, and was developed and refined by other research groups, Domen tells Physics World that in the present work, his team was able to demonstrate the approach’s effectiveness “more clearly than any former study”.

Finally, the researchers prevented an unwanted side reaction (the oxygen-reduction reaction) by encasing the rhodium co-catalysts for the hydrogen-producing reaction in a chromium-based protective shell.

Near-unity internal quantum efficiency

By combining these three strategies, the team demonstrated an EQE of up to 96% for their material when it was irradiated with light in the 350-360 nm range. This translates into an internal quantum efficiency (IQE), which is the fraction of absorbed photons that can be used to produce hydrogen, of near unity, which implies that the photocatalyst is almost perfect.

Domen and colleagues hope their strategies to improve the efficiency of SrTiO3 will also work for photocatalysts driven by visible light. They have published their results in Nature and Simone Pokrant of the University of Salzburg in Austria, who was not involved in this work, details their findings and their implications in a related Nature News and Views article.

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