14 June 2012
Scientists at the University of Washington, Seattle, Washington (US), are building a new fundamental understanding of reactions in which a proton and an electron are transferred together (proton-coupled electron transfer — or PCET). Taking a second look at previously undervalued chemical reactions on the surface of metal oxides, such as titanium dioxide and zinc oxide, the researchers have determined these reactions could be key to more efficient solar cells.
“Studies of the chemical reactivity of metal oxides have typically been understood in the context of solid-state physics models of their chemical bonding,” says James M. Mayer, Alvin L. and Verla R. Kwiram Professor of Chemistry at the University of Washington’s Department of Chemistry and coauthor of the paper “Titanium and Zinc Oxide Nanoparticles Are Proton-Coupled Electron Transfer Agents” recently published in Science. The study proposes a different model for certain kinds of processes, based on the idea that protons are being transferred as well — a new perspective that could lead the solar community down a much unexplored path toward more efficient energy utilization.
Prof. Mayer believes that for chemical reactions that happen at the surface of metal oxides, “considering the protons is an important piece of the puzzle that has not received much attention.” Referring to solar research in particular, he says, “It is also known that protons affect devices such as dye-sensitized solar cells, and maybe our work will lead to some atomic-level understanding of this effect and therefore how to improve the device efficiencies.”
The PCET expert has been studying the coupled transfers of electrons in protons in other contexts, including biological enzymes, vitamin C and synthetic oxidation catalysts. But, he says, “The more I learned about metal oxide chemistry — particularly from my colleague and collaborator Professor Daniel Gamelin — the more convinced I became that the concepts we had developed would apply to oxide surfaces.”
Granted that in some surface reactions the transfer process includes coupled electrons and protons, then how exactly can solar researchers use this newfound perspective effectively to optimize the efficiency of a solar cell? “If I knew the answer to that I would be doing it,” Prof. Mayer exclaims. He simply has not figured that out yet. “This is one of the exciting parts of doing fundamental science, that you do not know how new ideas are going to be used. Maybe we are just not smart enough to see this yet.” The chemist says he has talked to “more than a dozen significant players in the field” who all agree that “this is an area that is important and has been ignored.”
Encouraged by the supportive reactions of fellow scientists, Prof. Mayer has “a lot of ideas” for how to continue this particular inquiry. “We are probing, for instance, whether particles with many electrons react differently from particles with one electron — and similarly for one or more protons,” he says, adding that typically, researchers had not been able to control the electrons, protons and net charges and thus could not see how they affect reactivity. “Maybe the most exciting avenue is our tests to see if particles with multiple electrons and protons can transfer these essentially together to a substrate, as such reactions are relevant to the production of fuels from sunlight.”
Photo: Coated titanium dioxide nanocrystal; Joel Schrauben/James Mayer, University of Washington.
Written by Sandra Henderson, Research Editor, Solar Novus Today