In their latest experiments with semiconductor nanocrystals as light absorbers, physicists led by Professor Jochen Feldmann (Ludwig-Maximilians-Universität München, LMU Munich), in collaboration with a team of chemists under the direction of Professor Andrey Rogach (City University of Hong Kong), have succeeded in significantly increasing the yield of hydrogen produced by the photocatalytic splitting of water.
The crucial innovation, reported in the latest issue of the journal Nature Materials, is the use of a so-called molecular shuttle to markedly improve the mobility of charge carriers in their reaction system.
The apparent quantum yield and the formation rate under 447 nm laser illumination exceeded 53% and 63 mmol g−1 h−1, respectively. The fast hole transfer confers long-term photostability on the system and opens new pathways to improve the oxidation side of full water splitting.—Simon et al.
The amount of solar radiation that reaches the Earth in a year exceeds current annual energy needs by more than 10,000-fold; however, it is not yet possible to store sufficiently high amounts of solar energy in an efficient way. One approach is to utilize incoming solar radiation for the photocatalytic generation of molecular hydrogen (H2) from water.
When a quantum of light (a “photon”) with sufficient energy excites a semiconductor nanocrystal, it produces a negative charge (electron) and a positive charge (hole). Photocatalytic synthesis of hydrogen gas from water requires the transfer of electrons to the hydrogen, while the holes interact with the oxygen or are scavenged by other molecules.
However, before any of this can happen, the photogenerated electrons and holes must be quickly separated from each other. If the semiconducting nanocrystals are decorated with nanoparticles of a metal catalyst—such as the precious metal platinum—the electron can rapidly transfer to the metal and hydrogen production ensues.
However, unless the positively charged holes are effectively removed, they will accumulate and eventually bring H2 synthesis to a halt.
One problem for an efficient removal of holes is the need for polar molecules ot be attached to the nanocrystals as surface ligands in order to make the nanocrystals water-soluble. By doing so, however, the resulting “ligand forest” of the attached polar molecules makes it difficult for the holes to interact with water or larger scavenger molecules.
An analogy is the delivery of airline passengers to their final destination. Spatial constraints make it impossible for the aircraft to convey its passengers directly to their hotels in town. Instead, smaller and more maneuverable carriers, such as the shuttle buses, are used for the short last stage of the trip.
In a similar way, the research teams in Munich and Hong Kong hit on the idea of using one of the smallest constituents of their system—the hydroxyl ion formed by the dissociation of water—to penetrate the ligand forest, collect the holes from the surface of the crystals and transport them to a larger acceptor molecule.
The concentration of this molecular shuttle in the system can be easily controlled by altering the pH of the solution. Indeed, raising the pH of the solution drastically increases the rate of hydrogen production.
I was amazed the first time I tried it. As soon as I increased the pH I could see, with the naked eye, bubbles of hydrogen rising to the surface.—Thomas Simon, a PhD student at Professor Feldmann’s chair
The new system also has other advantages besides the increase in yield. First, its long-term stability could be markedly improved. Furthermore, it turns out that the costly platinum catalyst can be replaced by nickel, a far less expensive metal.
The discovery of this new mechanism could lead to entirely new approaches to the photocatalytic production of hydrogen.— Dr. Jacek Stolarczyk, head of the Photocatalysis group, chair of Photonics and Optoelectronics (PhOG) at LMU
Thomas Simon, Nicolas Bouchonville, Maximilian J. Berr, Aleksandar Vaneski, Asmir Adrović, David Volbers, Regina Wyrwich, Markus Döblinger, Andrei S. Susha, Andrey L. Rogach, Frank Jäckel, Jacek K. Stolarczyk and Jochen Feldmann (2014) “Redox shuttle mechanism enhances photocatalytic H2 generation on Ni-decorated CdS nanorods,” Nature Materials doi: 10.1038/nmat4049