Effect of the Degree of Inversion on the Photoelectrochemical Activity of Spinel ZnFe2O4

Physicochemical properties of spinel ZnFe2O4 (ZFO) are known to be strongly affected by the distribution of the cations within the oxygen lattice. In this work, the correlation between the degree of inversion, the electronic transitions, the work function, and the photoelectrochemical activity of ZFO was investigated. By room-temperature photoluminescence measurements, three electronic transitions at approximately 625, 547, and 464 nm (1.98, 2.27, and 2.67 eV, respectively) were observed for the samples with different cation distributions. The transitions at 625 and 547 nm were assigned to near-band-edge electron-hole recombination processes involving O(2-)2p and Fe(3+)3d levels. The transition at 464 nm, which has a longer lifetime, was assigned to the relaxation of the excited states produced after electron excitations from O(2-)2p to Zn(2+)4s levels. Thus, under illumination with wavelengths shorter than 464 nm, electron-hole pairs are produced in ZFO by two apparently independent mechanisms. Furthermore, the charge carriers generated by the O(2-)2p to Zn(2+)4s electronic transition at 464 nm were found to have a higher incident photon-to-current efficiency than the ones generated by the O(2-)2p to Fe(3+)3d electronic transition. As the degree of inversion of ZFO increases, the probability of a transition involving the Zn(2+)4s levels increases and the probability of a transition involving the Fe(3+)3d levels decreases. This effect contributes to the increase in the photoelectrochemical efficiency observed for the ZFO photoanodes having a larger cation distribution.