Molecular hydrogen (H₂) is a clean-burning fuel that can be produced from protons (H⁺) in the reductive half-reaction of artificial photosynthesis systems. One of the most prominent strategies for light-driven proton reduction features a multicomponent solution with a light absorbing molecule (chromophore) that transfers electrons to a catalyst that reduces protons. However, these solution systems often use nonaqueous solvents, and always have short lifetimes from decomposition of the chromophore over a period of hours. This difficulty has led to more complicated architectures that separate the sites of light absorption and proton reduction.

Semiconductor nanocrystals (NCs) are promising alternative chromophores for light driven proton reduction. Compared to traditional organic or organometallic chromophores, NCs have superior photostability, larger absorption cross-sections over a broad spectral range, orders of magnitude longer excited state lifetimes, electronic states and associated optical properties that vary with NC size, and the capacity to deliver multiple electrons with minimal structural perturbations. Heterostructures combining NCs with traditional precious metal nanoparticle proton reduction catalysts, or with iron-hydrogenases, have produced efficient proton reduction catalysis in solution. However, small-molecule catalysts in conjunction with NCs have given only modest H₂ production.

We report here a system that provides light-driven H₂ production with exceptional longevity, maintaining its high activity with no decrease for over two weeks using water as solvent.

—Han et al.

The work was done by graduate students Zhiji Han and Fen Qiu, as part of a collaboration between chemistry professors Richard Eisenberg, Todd Krauss, and Patrick Holland, which is funded by the US Department of Energy.

One disadvantage of current methods of photocatalytic hydrogen production has been the lack of durability in the light-absorbing material, but the Rochester scientists were able to overcome that problem by incorporating nanocrystals.

Organic molecules are typically used to capture light in photocatalytic systems. The problem is they only last hours, or, if you're lucky, a day. These nanocrystals performed without any sign of deterioration for at least two weeks.

—Todd Krauss

The process developed by Holland, Eisenberg, and Krauss is similar to other photocatalytic systems; they needed a chromophore (the light-absorbing material); a catalyst to combine protons and electrons; and a solution, which in this case is water. Krauss, an expert in nanocrystals, provided cadmium selenide (CdSe) quantum dots (nanocrystals) as the chromophore. Holland, whose expertise lies in catalysis and nickel research, supplied a nickel catalyst (nickel nitrate). The nanocrystals were capped with DHLA (dihydrolipoic acid) to make them water-soluble.

For their studies, they used ascorbic acid as the sacrificial electron donor (due to the complexity in building and optimizing a complete artificial photosynthesis system, when studying the reductive half-reaction it is common for a sacrificial electron donor to be used to optimize catalysts for hydrogen production). The NCs and the nickel catalyst were placed in water to form a solution which was then irradiated (λ = 520 nm). Under appropriate conditions, the system continued to produce H₂ at a constant rate for more than 360 hours. A control experiment without added Ni₂₊ yielded no significant H₂ production.

They hypothesized that the catalytic system functions through light absorption by the CdSe nanocrystal, electron transfer to the catalyst, and then proton reduction by the catalyst.

A light-driven system for the photogeneration of hydrogen that consists of simple components containing only Earth-abundant elements could have a significant impact on the sustainable production of chemical fuels. Further, the robustness of the system may be generalizable to other nanoparticle systems, such as Type II NCs and dot-in-rod NCs, which are better engineered for charge separation. When considering the efficiency of this system in a real-world context, further improvements could be made by adding light-harvesting components that absorb more of the solar spectrum, since with NC(540) only about 25% of the available solar flux is absorbed. Nonetheless, this particular NCDHLA-Ni system exhibits high activity for proton reduction and impressive durability, which suggests that it could also serve as a valuable component in complete artificial photosynthetic water splitting systems for light-to-chemical energy conversion.

—Han et al.

While the researchers say the commercial implementation of their work is years off, Holland points out that an efficient, low-cost system would have uses beyond energy, in any industry that requires large amounts of hydrogen such as pharmaceuticals and fertilizers.

Resources

• Zhiji Han, Fen Qiu, Richard Eisenberg, Patrick L. Holland, and Todd D. Krauss (2012) Robust Photogeneration of H₂ in Water Using Semiconductor Nanocrystals and a Nickel Catalyst. Science doi: 10.1126/science.1227775