Great scientific leaps often occur when new results come along that just can’t fit into the prevailing models and theories. A recent study in PNAS surely fits this description, challenging the current view of photocatalysis using recent data looking at carrier dynamics in titania-metal composites. Although replication studies and more data will be needed to confirm the new ideas presented, this is an exciting alternative view of photocatalysis that could inspire new classes of materials to improve performance and reduce costs.
Photocatalysis is a primary method used to convert solar energy to chemical fuel, such as hydrogen. If fuel cells end up being viable, for example, we will need methods of creating and storing the hydrogen that is used as the input fuel in many types of cells (the infamous hydrogen economy!). One process to do this is called photocatalytic water splitting, by which water is split into hydrogen and oxygen. Once the water is split, we can capture the hydrogen and store it for future use as fuel. But water is happy how it is, so we need to give it energy to motivate it to split up into its constituent parts.That’s where the choice of materials and catalysts for the water splitting apparatus comes into play.
If we create a device made of titania (a semiconductor) immersed with water, then incoming light will excite electrons in the titania. These electrons then have the energy to initiate the water splitting reaction! However, when researchers first tried this, they found that titania alone didn’t promote much water splitting and subsequent hydrogen production. When they added a metal to the titania surface, though, suddenly hydrogen production skyrocketed!
The answer to the question you’re probably thinking right now (why?!) is still under much debate, but the current consensus is that the metal traps electrons and prevents them from recombining with holes (areas of positive charge left behind by the excited electron). This can be a problem because the electron needs to use its energy given by photons from the sun to induce the water splitting reaction. However, the electron can also lose this energy by recombining with a hole in the titania (recombination). If this recombination process occurs faster than the water redox reaction, then we’re out of luck in terms of water splitting. So researchers thought that adding a metal, known as a catalyst in the water splitting reaction, provides a trap for the electron to keep it away from the hole, letting it ‘live’ long enough to reduce a proton (H+) to a hydrogen atom. You can see this in Figure A below, where the Au is the gold catalyst trapping the electron on top of the titania (TiO2) surface.This has been the theory guiding researchers for some time, which has had a great influence on what types of materials are explored and how scientists think about improving hydrogen production rates on the titania surface (trying different metals, different geometries, etc.). But now, Joo et al have reported new data suggesting that this whole way of looking at photocatalysis is wrong! Instead, they propose a new method, shown in Figure B above. They state that protons (H+) are reduced by the excited electron in the titania, and the metal is required for atomic hydrogen recombination (H* to H2). Joo et al were inspired by their newly collected data that showed that the rate at which electrons and holes recombined in titania remained unchanged no matter if there was a metal on the titania surface or not (shown above, analyzed using time-resolved fluorescence spectroscopies, for the experts out there). The figure above shows this rate (time on the x-axis) for various amounts of metal – but all the lines are identical, you can barely tell them apart! This flies in the face of the model I presented above, which would predict that recombination rates would greatly decrease in the presence of a metal that supposedly traps electrons. It’s amazing that so simple a measurement revealed such a fundamental flaw in the prevailing view of catalysis!
So something else is going on here…the authors performed several more experiments to determine exactly what is dictating the rate of hydrogen production. To test their new hypothesis that metals are important for atomic hydrogen recombination, not the reduction of protons, they performed an experiment to distinguish between these two effects. Basically, they replaced the metal with another material that could only provide one of the two purported mechanisms: electron trapping or atomic hydrogen recombination.First, they replaced the metal with conductive carbon dispersed across the TiO2 surface – this should be able to trap electrons but not assist in hydrogen recombination. And, indeed, the authors found minimal hydrogen production with this setup, indicating that electron trapping is not vital (blue bar in figure above).
Second, they then added a nickel oxide to the TiO2 surface. The electronic structure of nickel oxide makes it such that electrons cannot transfer to it from TiO2, thus making it impossible to be an electron trapper. Also, previous research had shown that atomic hydrogen recombination is an exothermic reaction on nickel oxide, making it viable for the author’s alternative hypothesis. Again supporting their idea, placing nickel oxide on titania did indeed lead to significant hydrogen production (purple bar in figure above)!
Based on the figure, gold (Au) and platinum (Pt) still demonstrate better hydrogen production than nickel oxide (black and red bars), but these results now provide important insight into what Au and Pt are actually doing. Proton reduction, creating a hydrogen atom, appears to be occurring in TiO2 at a fast enough rate to out-compete the electron-hole recombination rate. But the metals seem to be playing a big role in promoting the recombination of two lone hydrogen atoms into hydrogen gas (H2), the final result that we can use as fuel.
These results will definitely be a surprise to the scientific community. Most importantly, we may have been designing catalysts optimized for the completely wrong part of the reaction! New research into optimizing hydrogen atom recombination could lead to significantly improved photocatalytic water splitting performance and make this method of chemical fuel production more viable.
Joo, J., Dillon, R., Lee, I., Yin, Y., Bardeen, C., & Zaera, F. (2014). Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H2 Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1405365111