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Download filePromoting Active Electronic States in LaFeO3 Thin-Films Photocathodes via Alkaline-Earth Metal Substitution
journal contribution
posted on 2020-06-30, 13:36 authored by Xin Sun, Devendra Tiwari, David J. FerminThe effects of alkaline-earth
metal cation (AMC; Mg2+, Ca2+, Sr2+, and Ba2+) substitution on the photoelectrochemical properties
of phase-pure LaFeO3 (LFO) thin-films are elucidated by
X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), diffuse
reflectance, and electrochemical impedance spectroscopy (EIS). XRD
confirms the formation of single-phase cubic LFO thin films with a
rather complex dependence on the nature of the AMC and extent of substitution.
Interestingly, subtle trends in lattice constant variations observed
in XRD are closely correlated with shifts in the binding energies
of Fe 2p3/2 and O 1s orbitals associated with the perovskite
lattice. We establish a scaling factor between these two photoemission
peaks, unveiling key correlation between Fe oxidation state and Fe–O
covalency. Diffuse reflectance shows that optical transitions are
little affected by AMC substitution below 10%, which are dominated
by a direct bandgap transition close to 2.72 eV. Differential capacitance
data obtained from EIS confirm the p-type characteristic of pristine
LFO thin-films, revealing the presence of sub-bandgap electronic state
(A-states) close to the valence band edge. The density of A-states
is decreased upon AMC substitution, while the overall capacitance
increases (increase in dopant level) and the apparent flat-band potential
shifts toward more positive potentials. This behavior is consistent
with the change in the valence band photoemission edge. In addition,
capacitance data of cation-substituted films show the emergence of
deeper states centered around 0.6 eV above the valence band edge (B-states).
Photoelectrochemical responses toward the hydrogen evolution and oxygen
reduction reactions in alkaline solutions show a complex dependence
on alkaline-earth metal incorporation, reaching incident-photon-to-current
conversion efficiency close to 20% in oxygen saturated solutions.
We rationalize the photoresponses of the LFO films in terms of the
effect sub-bandgap states on majority carrier mobility, charge transfer,
and recombination kinetics.
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Keywords
electrochemical impedance spectroscopyeffect sub-bandgap statesoxygen reduction reactionslatticealkaline-earth metal incorporationFe oxidation stateXPSsolutionphase-pure LaFeO 3LFOA-statevalence band edgeX-ray photoemission spectroscopyeVcation-substituted films showXRDshifttransitiondependenceLaFeO 3 Thin-Films PhotocathodesDifferential capacitance dataAlkaline-Earth Metal Substitutionmajority carrier mobilityincident-photon-to-current conversi...AMC substitutionreflectanceEISalkaline-earth metal cationvalence band photoemission edgethin-film