The discovery of the effect of quantum confinement in a 3D-ordrered microporous (3DOM) structure photocatalyst has enabled the production of hydrogen under a visible light source. A research team led by researchers from China and Germany made the discovery, which could offer an option for addressing energy and environmental challenges.

The scientists said that the ability to produce green hydrogen from solar water splitting is an attractive possibility, due to the high energy density of hydrogen fuel. Hydrogen produced from solar water splitting avoids carbon emissions. And hydrogen has industrial purposes beyond use in fuel cells for electricity.

Quantum confinement refers to changes in optical and electronic properties — such as energy levels and bandgaps — when the size of the material is reduced to proportions at the nanoscale.

While the typical photocatalyst for solar water splitting can absorb UV light only from the solar spectrum, which accounts for about 4% of the energy from sunlight, the researchers used bismuth vanadate (BiVO₄). The metal oxide photocatalyst is responsive both to UV and visible light and can absorb up to 30% of the energy in the solar spectrum.

As a result, BiVO₄ has received considerable attention in research pursuits. The larger size of its surface area, high light absorption, and suppressed charge recombination owe to the favorable photocatalytic activities of the structure.

The team discovered that in the water splitting process under visible light, the amount of oxygen that the 3DOM BiVO₄ catalyst produced was almost 2× that produced by BiVO₄, which it described as “plate-like.” Furthermore, the photocatalyst exhibited higher anodic photocurrent density than the plate-like form.

The results showed that 3DOM BiVO₄ had a higher photocatalysis efficiency.

The researchers then made a discovery based on the elevation of the conduction band of the 3DOM BiVO4. “We discovered that quantum confinement arising from the ultrathin, crystalline wall of 3DOM BiVO4 raised its conduction band,” said Ng Yun Hau, associate professor in the City University of Hong Kong’s School of Energy and Environment. “It enables the photocatalytic proton reduction to hydrogen under visible-light illumination, allowing hydrogen to be generated from water splitting.”

Hau further explained that, in general, BiVO₄ cannot produce hydrogen because of its position of the conductive band. “Now thanks to the quantum confinement effect, which raised its conduction band, hydrogen can be produced. This is also the first time that the quantum confinement effect was found in 3DOM BiVO₄.”

Even without using a cocatalyst — a substance that facilitates the function of a catalyst, can provide accumulating sites for photogenerated charges, and can promote charge separation — the team found that 3DOM BiVO₄ still produced hydrogen from solutions under visible light illumination. The plate-like BiVO4 showed only negligible hydrogen production. It also applied time-resolved microwave conductivity and other advanced techniques to investigate the BiVO₄ photocatalyst in 3DOM and plate-like structures. The team found that compared with the plate-like structure, the photocatalysts had about an 18×-greater charge carrier lifetime, as well as about 9×-longer effective diffusion length. These qualities enhance the efficiency of photocatalysis, the team said.

According to the team, its study is a step toward understanding charge transport in metal oxide semiconductors and highly ordered porous structures. Its next step is to split wastewater and explore methods to scale up photocatalytic systems.

The research was published in ACS Energy Letters (www.doi.10.1021/acsenergylett.1c01454).