Structure, band offsets and photochemistry at epitaxial α-Cr2O3/α-Fe2O3 heterojunctions

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.