Promoting Active Electronic States in LaFeO₃ Thin-Films Photocathodes via Alkaline-Earth Metal Substitution

Abstract

The effects of alkaline-earth metal cation (AMC: Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ ) 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 towards more positive potentials. This behaviour 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 centred around 0.6 eV above the valence band edge (B-states). Photoelectrochemical responses towards 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 rationalise the photoresponses of the LFO films in terms of the effect sub-bandgap states on majority carrier mobility, charge transfer and recombination kinetics.

High Interfacial Hole‐Transfer Efficiency at GaFeO3 Thin Film Photoanodes

“…Furthermore, the Ga 2p 3/2 peak position at 1118 eV is characteristic of native gallium oxide, confirming that Ga is in a +3 oxidation state. [36,37] The main feature of the O 1s spectrum ( Figure 2c) is centered at 530.01 eV, which is assigned to lattice bound oxygen, [13] while the other contributions are linked to surface hydroxyl and carbonylated groups. [38] Fe 2p core-level ( Figure 2d) is significantly more complex to rationalize due to contribution from various phenomena such as multiple-splitting, multiple oxidation states, and charge transfer effects.…”

“…As a first approximation, the most intense component in 2p 3/2 is at 710.8 eV, with a 13.5 eV splitting from the 2p 1/2 component, which is consistent with a Fe+3 oxidation state. [13,17] Other higher binding energy components can be linked to higher oxidation states; however, this would require further analysis of other Fe core-levels. [39] Interestingly, the observed surface Ga:Fe ratio of 1.17 is very close to the bulk ratio obtained from EDX spectrum (1.19, see Figure S1, Supporting Information) and XRD refined site occupancy (1.15, Figure 1a).…”

“…GFO Thin Film Preparation: GFO thin films were synthesized following a modified sol-gel method. [13] First, 0.5 m Ga(NO 3 ) 3 •6H 2 O aqueous solution (1 mL) and 0.5 m Fe(NO 3 ) 3 •9H 2 O aqueous solution (1 mL) were mixed under strong stirring at 60 °C for 1 h. 1 m citric acid ethanolic solution (1 mL) and ethylene glycol (62.5 mL) were subsequently added, and the solution was further stirred overnight in a capped glass vial. The precursor solution was spin-coated onto clean F:SnO 2 (FTO) conductive glass at 3000 rpm for 30 s, followed by drying at 100 °C for 10 min and thermolyzing the metal-complex at 400 °C for 1 h, to deposit 1 layer of GFO.…”

“…[ 9 ] Other ferrite photoanodes include ZnFe 2 O 4 , MgFe 2 O 4 , and CuFe 2 O 4 , achieving external quantum efficiency values of close to 10% and photocurrent onset potentials ranging from 0.6 up to 1 V. [ 10,11 ] Ferrite photocathodes such as LaFeO 3 and YFeO 3 have shown very interesting photovoltages for the hydrogen evolution reaction, but their activity is limited by bulk and surface recombination losses. [ 12–17 ] Ferroelectric materials such as BiFeO 3 and Bi 2 FeCrO 6 show complex PEC properties that have been linked to ferroelectric domains. [ 18,19 ] For instance, BiFeO 3 exhibits both n‐ and p‐type conductivity, including above photovoltages larger than the bandgap.…”

Progress in Ternary Metal Oxides as Photocathodes for Water Splitting Cells: Optimization Strategies

“…In addition, an increase of the p‐type character upon doping can be understood by both the appearance of empty states above the valence band and the flattening of the bands in the CB, which is indicative of an increase in the electron effective mass. Another work by Sun et al [ 213 ] focused on the effect of doping with divalent alkaline‐earth metal cations (Mg, Ca, Sr, and Ba) in LaFeO 3 prepared by sol‐gel. The authors claimed that the increase in the photocurrent is linked to the attenuation of states located around 100–200 meV above the valence band edge that are linked to intrinsic defects (cation vacancies) and act as hole‐traps.…”