Researchers have devised a more sustainable alternative to the reliance on fossil fuels for hydrogen production: a photocatalyst based on metal epsilon-near-zero (ENZ) metamaterials. These light-trapping materials mimic natural photosynthetic water splitting.

Nanostructured metals that convert solar electrons into intense, wave-like plasmon resonances are of interest for hydrogen production. The high-speed metal plasmons help transfer carriers to catalytic sites before they relax and reduce catalytic efficiency.

The ENZ metamaterials grow with random, fractal needles similar to a tiny pine tree. The propagation of light Metallic nanostructures that slow down light dramatically can triple the efficiency of solar-based hydrogen fuel generation. Source: King Abdullah University of Science and Technologyslows to a near standstill inside the cavities formed by the protruding needles, allowing the material to exploit the entire solar spectrum.

A major challenge for the researchers from King Abdullah University of Science and Technology, Saudi Arabia, was to test various fabrication variables in order to arrive at the correct configuration of needle structures for efficient reactions. Crystals with high purity were also required to optimize the use of semiconducting titanium dioxide as a substrate to collect hot electrons. An additional requirement: the concentration and position of platinum nanoparticles used to catalytically split water molecules needed to be precisely controlled, depositions that are difficult with ENZ’s complex geometry.

Tests with the final system demonstrated that the ENZ photocatalyst used broadband light to generate hot carriers within a narrow 10-nm interfacial region for an overall 300 percent gain in efficiency relative to that of the best previously reported plasmonic-based photocatalysts for the dissociation of hydrogen. The new photocatalyst achieved a hydrogen production rate of 9.5 µmol h/cm².