| AVS 60th International Symposium and Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS+AS-TuM |
| Session: | Synthesis, Structure and Characterization of Oxides |
| Presenter: | S.E. Chamberlin, Pacific Northwest National Laboratory |
| Authors: | S.E. Chamberlin, Pacific Northwest National Laboratory Y. Wang, Pacific Northwest National Laboratory T.C. Kaspar, Pacific Northwest National Laboratory M.E. Bowden, Pacific Northwest National Laboratory A.W. Cohn, University of Washington D.R. Gamelin, University of Washington P.V. Sushko, University College London S.A. Chambers, Pacific Northwest National Laboratory |
| Correspondent: | Click to Email |
There is widespread interest in discovering materials that can effectively harvest sunlight in the visible region of the solar spectrum in order to drive chemical processes on surfaces. Hematite (α-Fe2O3) has received renewed interest recently as the active photoanode in photoelectrochemical water splitting to store solar energy as H2 [1]. Hematite is very abundant, has a narrow bandgap of 2.1 eV, and is stable in aqueous and oxidizing environments. However, it has extremely poor electron and hole mobilities, which results in short hole diffusion lengths and ultrafast recombination of photogenerated e- - h+ pairs. Additionally, it does not efficiently absorb photons in the 500-600 nm range where the solar spectrum is most intense. The utility of hematite could therefore be significantly increased by red shifting its band gap to harvest more of the solar spectrum.
Eskolaite (α-Cr2O3) has a band gap of 3.3 eV, but alloying Cr and Fe to make α-(Fe1-xCrx)2O3 solid solutions as epitaxial films has been found to result in a reduction in gap down to as low as ~1.7 eV at x = ~0.5 [2,3]. We examine the optical absorption in α-(Fe1-xCrx)2O3 thin films grown by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) on α-Al2O3(0001) substrates and explain the observed excitations, and the nature of the band gap dependence on x, through first principles calculations. Photoconductivity (PC) measurements show that the onset of photocurrent decreases by nearly 0.5 eV when x is increased from 0 to ~0.4. However, for x > ~0.6, the films are not sufficiently conductive for PC to be measured.
Karelianate (α-V2O3) is weakly metallic at room temperature, so α-(Fe1-xVx)2O3 films may overcome the conductivity challenges of the α-(Fe1-xCrx)2O3 system. Substitutional V impurities have also recently been predicted to lower the band gap of α-Fe2O3 further than that of Cr-doped α-Fe2O3 [4]. Heteroepitaxial thin films of α-(Fe1-xVx)2O3 were deposited on α-Al2O3(0001) substrates by OPA-MBE. Film quality was monitored in situ by reflection high energy electron diffraction (RHEED). In situ x-ray photoemission spectroscopy (XPS) was utilized to characterize the charge states of the cations. Film crystallinity and lattice parameters were determined ex situ by high resolution x-ray diffraction (HRXRD). The effects of the doping concentration on the conductivity and optical properties will be discussed.
[1] K. Sivula et al., Chemsuschem 4, 432 (2011).
[2] H. Mashiko, et al., Appl. Phys. Lett. 99, 241904 (2011).
[3] S.E. Chamberlin et al., J. Phys. Chem. Lett., submitted (2013).
[4] Z.D. Pozun and G. Henkelman, J. Chem. Phys. 134, 224706 (2011).