Structure, band offsets and photochemistry at epitaxial α-Cr2O3/α-Fe2O3 heterojunctions |
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Authors: | SA Chambers JR Williams AG Joly SJ Pennycook |
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Institution: | a Pacific Northwest National Laboratory, Fundamental Science Directorate, 3335 Q Avenue, Richland 99352 WA, USA b Oak Ridge National Laboratory, PO BOX 2008, Oak Ridge 37831, TN, USA |
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Abstract: | We test the hypothesis that electron-hole pair separation following light absorption enhances photochemistry at oxide/oxide heterojunctions which exhibit a type II or staggered band alignment. We have used hole-mediated photodecomposition of trimethyl acetic acid chemisorbed on surfaces of heterojunctions made from epitaxial α-Cr2O3 on α-Fe2O3(0001) to monitor the effect of UV light of wavelength 385 nm (3.2 eV) in promoting photodissociation. Absorption of photons of energies between the bandgaps of α-Cr2O3 (Eg = 4.8 eV) and α-Fe2O3 (Eg = 2.1 eV) is expected to be strong only in the α-Fe2O3 layer. The staggered band alignment should then promote the segregation of holes (electrons) to the α-Cr2O3 (α-Fe2O3) layer. Surprisingly, we find that the α-Cr2O3 surface alone promotes photodissociation of the molecule at hν = 3.2 eV, and that any effect of the staggered band alignment, if present, is masked. We propose that the inherent photoactivity of the α-Cr2O3(0001) surface results from the creation of bound excitons in the surface which destabilize the chemisorption bond in the molecule, resulting in photodecomposition. |
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Keywords: | Photocatalysis Heterojunctions Epitaxial |
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