The autonomous plasmonic solar water splitter, which was operated with 1 M potassium borate electrolyte (pH 9.6) under various illumination conditions, showed impressive photosynthetic hydrogen and oxygen production for a device in which all active charge carriers originate from the decay of surface plasmons in the gold nanorods. Its stability when illuminated with visible light (λ > 410 nm) is exceptional, in contrast to devices based on narrow-bandgap semiconductors, which commonly photocorrode rapidly. Hydrogen production was clearly observable after ~2 h.

—Mubeen et al.

For this experiment, gold nanorods were capped with a layer of crystalline titanium dioxide (TiO₂) decorated with platinum nanoparticles, which functions as the hydrogen evolution catalyst, and set in water. A cobalt-based oxidation catalyst (Co-OEC) was deposited on the exposed portions of the gold nanorods to enhance oxygen gas evolution. The TiO₂ acts as an electron filter and as support for the platinum nanoparticles that serve as the hydrogen evolution catalyst.

Structure and mechanism of operation of the autonomous plasmonic solar water splitter. (a) Schematic of the cross-section of an individual photosynthetic unit showing the inner gold nanorod, the TiO2 cap decorated with platinum nanoparticles, which functions as the hydrogen evolution catalyst, and the Co-OEC material deposited on the lower portion of the gold nanorod. (b) Corresponding transmission electron micrograph (left) and magnified views of the platinum/TiO2 cap (top right) and the Co-OEC (bottom right). (c) Energy level diagram superimposed on a schematic of an individual unit of the plasmonic solar water splitter, showing the proposed processes occurring in its various parts and in energy space. CB, conduction band; VB, valence band; EF, Fermi energy. Click to enlarge. Source: Mubeen et al.

Though still in early stages, the research offers the promise to convert sunlight into energy using a process based on metals that are more robust than many of the semiconductors used in conventional methods.

It is the first radically new and potentially workable alternative to semiconductor-based solar conversion devices to be developed in the past 70 years or so.

—Martin Moskovits, professor of chemistry at UCSB

In conventional photoprocesses, a technology developed and used over the last century, sunlight hits the surface of semiconductor material, one side of which is electron-rich, while the other side is not. The photon, or light particle, excites the electrons, causing them to leave their postions, and create positively-charged “holes.” The result is a current of charged particles that can be captured and delivered for various uses, including powering lightbulbs, charging batteries, or facilitating chemical reactions.

In the technology developed by Moskovits and his team, it is not semiconductor materials that provide the electrons and venue for the conversion of solar energy, but a forest of gold nanorods.

When nanostructures, such as nanorods, of certain metals are exposed to visible light, the conduction electrons of the metal can be caused to oscillate collectively, absorbing a great deal of the light. This excitation is called a surface plasmon.

—Martin Moskovits

As the hot electrons in these plasmonic waves are excited by light particles, some travel up the nanorod, through a filter layer of crystalline titanium dioxide, and are captured by platinum particles. This causes the reaction that splits hydrogen ions from the bond that forms water. Meanwhile, the holes left behind by the excited electrons head toward the cobalt-based catalyst on the lower part of the rod to form oxygen.

According to the study, hydrogen production was clearly observable after about two hours. Additionally, the nanorods were not subject to the photocorrosion that often causes traditional semiconductor material to fail in minutes.

Although high for a plasmonic system, the reported solar-to-hydrogen efficiency (~0.1%) is low for practical use, but on par with the values reported for early water splitters based on electron–hole pair production in semiconductors. Straightforward structural improvements to the device we describe can lead to significant efficiency improvements. For example, the hydrogen evolution catalyst (platinum nanoparticles on TiO₂) currently covers a very small fraction of the nanorods’ surface. This can be improved by increasing the spacing between the nanorods to allow a larger fraction of the nanorods’ surface to be processed, with a simultaneous increase in the nanorods’ length to ensure that the array’s plasmonic absorption remains high. On the other hand, the operational lifetime of our plasmonic device already exceeds that of the most efficient water splitters based on semiconductors.

—Mubeen et al.

Research in this study was also performed by postdoctoral researchers Syed Mubeen and Joun Lee; grad student Nirala Singh; materials engineer Stephan Kraemer; and chemistry professor Galen Stucky.

Resources

• Syed Mubeen, Joun Lee, Nirala Singh, Stephan Krämer, Galen D. Stucky & Martin Moskovits (2013) An autonomous photosynthetic device in which all charge carriers derive from surface plasmons. Nature Nanotechnology doi: 10.1038/nnano.2013.18