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Helpful holes in hematite
Electrons are well-known (negative) charge carriers, indispensable in our daily lives, but they do not play this role alone. When an electron leaves its assigned place, it leaves behind a hole that can effectively behave like a positive charge carrier, provided that the electron and hole remain separated and do not recombine. In modern semiconductor electronics, holes are important charge carriers, without which devices like batteries, capacitors, fuel cells, solar cells, and PEC could not operate. PEC electrodes typically form electron-hole pairs when struck by sunlight. In PEC photoanodes made of hematite, the generated holes must diffuse to the semiconductor surface, where they can oxidize water and form oxygen.
Not all holes are created equalIn a recent study published in the «Journal of Physical Chemistry C», Empa researchers Artur Braun and Debajeet Bora and their colleagues from EPF Lausanne, the University of Basel, China and the US studied the nature of photoelectrically generated holes in a PEC that had been specially designed for gathering data while the cell is in operation («operando»). They recorded soft X-ray absorption spectra under simulated sunlight and in the dark and identified two new spectral signatures corresponding to two different hole transitions, an O 2p hole transition into the charge-transfer band and an Fe 3d-type hole transition into the upper Hubbard band. According to Braun, this is the first time that the electronic structure of a PEC photoanode has been analyzed while it was in real water splitting action, i.e. in contact with electrolyte, under anodic bias and illuminated by visible light. «The preparations for this extremely complex experiment took us three years», says Braun. «After all, soft X-ray spectroscopy works only in ultra-high vacuum, and photoelectrochemistry works only in liquids. Combining both was technically a great advancement. Yet, I would say we were very fortunate to discover the two electron holes in an operating PEC.»
Artur Braun | EurekAlert!
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