| AVS 59th Annual International Symposium and Exhibition | |
| In Situ Microscopy and Spectroscopy Focus Topic | Tuesday Sessions |
| Session IS+AS+SS+EN-TuM |
| Session: | In Situ Spectroscopic Studies of Catalysis and Gas-Solid Reactions |
| Presenter: | J.H. Guo, Lawrence Berkeley National Laboratory |
| Authors: | J.H. Guo, Lawrence Berkeley National Laboratory A. Braun, Empa, Swiss Federal Laboratories for Materials Science and Technology K. Sivula, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland D. Bora, Lawrence Berkeley National Laboratory J.F. Zhu, University of Science and Technology of China L. Zhang, University of Science and Technology of China M. Grätzel, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland E.C. Constable, University of Basel, Swtizerland |
| Correspondent: | Click to Email |
Hydrogen fuel generation by solar water splitting in photoelectrochemical cells (PEC) is one of the first steps in artificial photosynthesis and an essential part of the holy grail of solar energy conversion. Iron oxide, literally "rust", is an interesting PEC photoanode material because of its affordability, good stability, good spectral match of the solar spectrum, and yet controversial because of its poor electronic structure. At present, iron oxide is taking center stage as prospective PEC anode material.
PEC electrodes are typically semiconducting metal oxides to form electron-hole pairs when strucked by light. In the photoanodes such as hematite, the generated holes must diffuse to the iron oxide surface where they can oxidize water to oxygen. However, the electronic structure of iron oxide is such that the photogenerated holes tend to recombine and annihilate with the electrons before reaching the surface and performing the required chemical work on water splitting. Currently, researchers worldwide try to understand the peculiarities of iron oxide so as to invent strategies to improve this material.
The Advanced Light Source produces soft X-rays which are optimally suited to study the electronic structure of electrode materials and which can detect electron holes. But the holes needed for solar water splitting by iron oxide require an anodic electric bias plus the illumination. Moreover, the holes are transitional and quite elusive. Also, soft X-rays cannot easily peek into a PEC cell. The unique design of the in-situ cell at the ALS has overcome the burden [1-3]. Recently the experiment has been performed for studying, under in-situ and operando conditions, the hole generation in a specifically designed photoelectrochemical cell. The oxygen valence band signature was recorded while tuning the PEC relevant parameters, two different types of holes in the valence band near the Fermi energy are discovered [4].
References:
[1] "X-ray Emission Spectroscopy of Hydrogen Bonding and Electronic Structure of Liquid Water", J.-H. Guo et al., Phys. Rev. Lett. 89, 137402 (2002).
[2] “Electronic Structure of Cobalt Nanocrystals Suspended in Liquid”, H. Liu et al., Nano Lett. 7, 1919 (2007).
[3] “In situ soft X-ray absorption spectroscopy investigation of electrochemical corrosion of copper in aqueous NaHCO3 solution”, P. Jiang et al., Electrochem. Comm. 12, 820 (2010).
[4] “Direct Observation of Two Electron Holes in a Hematite Photoanode during Photoelectrochemical Water Splitting”, A. Braun et al., J. Phys. Chem. C 116, 16870 (2012).