The complex enzymes inside plants are impractical to manufacture, so researchers have developed photocatalysts that employ high-energy, hot electrons to cleave water molecules into hydrogen and oxygen gas. Recently, nanostructured metals that convert solar electrons into intense, wave-like plasmon resonances have attracted interest for hydrogen production. The high-speed metal plasmons help transfer carriers to catalytic sites before they relax and reduce catalytic efficiency.

An efficient hot carrier generation and collection requires, ideally, their generation to be enclosed within few tens of nanometers at the metal interface, but it is challenging to achieve this across the broadband solar spectrum.

Plasmonic systems have specific geometries that trap light only at characteristic frequencies. Some approaches try to combine multiple nanostructures to soak up more colors, but these absorptions take place at different spatial locations so the sun’s energy is not harvested very efficiently.”

—Andrea Fratalocchi, research leader

Fratalocchi and his team devised a new strategy using metal nanostructures known as epsilon-near-zero (ENZ) metamaterials that grow with random, fractal needles similar to a tiny pine tree. ENZ metamaterials are optical structures the properties of which emerge as the refractive index of the structure approaches zero. Inside the cavities formed by the protruding metal branches, the propagation of light slows to a near standstill. This enables the ENZ substance to squeeze all visible light colors to the same nanometer-scale locations.

Broadband ENZ plasmonic photocatalysis: general idea and sample fabrication. (a) Schematic illustration of a freestanding ENZ photocatalyst material. (b) Optical properties of the complex ENZ structure along the (x0,y0) plane. ENZ regions are illustrated in dark blue color. (c) Zoomed detail of (b), showing the motion of SPP waves excited by the light-matter interaction with impinging broadband photons. Panels d-e show the equivalent structures of panels b-c, respectively calculated by applying transformation optics. Panel d shows also the corresponding broadband squeezing of light in the points of positive curvatures of the material, where the equivalent ENZ regions exist. (f) STEM image of a fabricated sample, with a 2D cross section in yellow representing the structure of panel e. (g) SEM image and (h) naked eye image of the sample. (i) Broadband ENZ plasmonic photocatalysis reaction diagram. Tian et al. Click to enlarge.

However, optimizing the ENZ material for hydrogen generation proved a protracted process of months. Not every needle-like structure works the same way, which meant the team had to fine-tune all fabrication parameters to find the correct disorder for efficient reactions. Then, choosing semiconducting titanium dioxide as a substrate to collect hot electrons required crystals with extremely high purity. Finally, 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.

However, the result proved worth the perseverance: experiments revealed the ENZ photocatalyst used broadband light to generate hot carriers within a narrow 10-nm interfacial region for an overall 300% gain in efficiency.

Due to the possibility of controlling their absorption, the ENZ nanostructures are ideal candidates for solar-energy harvesting. We recently engineered an industrial prototype with impressive efficiency, which makes us very optimistic about the future possibilities of this technology.

—Andrea Fratalocchi

Resources

• Tian, Y., García de Arquer, F. P., Dinh, C.-T., Favrand, G., Bonifazi, M., … & Fratalocchi, A. (2017) “Enhanced solar-to-hydrogen generation with broadband Epsilon-Near-Zero nanostructured photocatalysts.” Advanced Materials 29, 1701165 doi: 10.1002/adma.201701165